CASCODE

The cascode is a two-stage amplifier composed of a transconductance amplifier followed by a current buffer. Compared to a single amplifier stage, this combination may have one or more of the following advantages: higher input-output isolation, higher input impedance, higher output impedance, higher gain or higher bandwidth. In modern circuits, the cascode is often constructed from two transistors (BJTs or FETs), with one operating as a common emitter or common source and the other as a common base or common gate. The cascode improves input-output isolation (or reverse transmission) as there is no direct coupling from the output to input. This eliminates the Miller effect and thus contributes to a much higher bandwidth.

History

The cascode (sometimes verbified to cascoding) is a universal technique for improving analog circuit performance, applicable to both vacuum tubes and transistors. The word "cascode" is a contraction of the phrase "cascade to cathode". It was first used in an article by F.V. Hunt and R.W. Hickman in 1939, in a discussion for application in low-voltage stabilizers.[1] They proposed a cascode of two triodes (first one with common cathode, the second one with common grid) as a replacement of a pentode.

Operation

Figure 1: N-channel cascode amplifier with resistive load (neglecting biasing details)

Figure 1 shows an example of cascode amplifier with a common source amplifier as input stage driven by signal source Vin. This input stage drives a common gate amplifier as output stage, with output signal Vout.

The major advantage of this circuit arrangement stems from the placement of the upper Field Effect Transistor (FET) as the load of the input (lower) FET's output terminal (drain). Because at operating frequencies the upper FET's gate is effectively grounded, the upper FET's source voltage (and therefore the input transistor's drain) is held at nearly constant voltage during operation. In other words, the upper FET exhibits a low input resistance to the lower FET, making the voltage gain of the lower FET very small, which dramatically reduces the Miller feedback capacitance from the lower FET's drain to gate. This loss of voltage gain is recovered by the upper FET. Thus, the upper transistor permits the lower FET to operate with minimum negative (Miller) feedback, improving its bandwidth.

The upper FET gate is electrically grounded, so charge and discharge of stray capacitance Cdg between drain and gate is simply through RD and the output load (say Rout), and the frequency response is affected only for frequencies above the associated RC time constant: τ = Cdg RD//Rout, namely f = 1/(2πτ), a rather high frequency because Cdg is small. That is, the upper FET gate does not suffer from Miller amplification of Cdg.

If the upper FET stage were operated alone using its source as input node (i.e. common-gate (CG) configuration), it would have good voltage gain and wide bandwidth. However, its low input impedance would limit its usefulness to very low impedance voltage drivers. Adding the lower FET results in a high input impedance, allowing the cascode stage to be driven by a high impedance source.

If one were to replace the upper FET with a typical inductive/resistive load, and take the output from the input transistor's drain (i.e. a common-emitter (CE) configuration), the CE configuration would offer the same input impedance as the cascode, but the cascode configuration would offer a potentially greater gain and much greater bandwidth.

Stability

The cascode arrangement is also very stable. Its output is effectively isolated from the input both electrically and physically. The lower transistor has nearly constant voltage at both drain and source and thus there is essentially "nothing" to feed back into its gate. The upper transistor has nearly constant voltage at its gate and source. Thus, the only nodes with significant voltage on them are the input and output, and these are separated by the central connection of nearly constant voltage and by the physical distance of two transistors. Thus in practice there is little feedback from the output to the input. Metal shielding is both effective and easy to provide between the two transistors for even greater isolation when required. This would be difficult in one-transistor amplifier circuits, which at high frequencies would require neutralization.

Biasing

As shown, the cascode circuit using two "stacked" FET's imposes some restrictions on the two FET's—namely, the upper FET must be biased so its source voltage is high enough (the lower FET drain voltage may swing too low, causing it to leave saturation). Insurance of this condition for FET's requires careful selection for the pair, or special biasing of the upper FET gate, increasing cost.

The cascode circuit can also be built using bipolar transistors, or MOSFETs, or even one FET (or MOSFET) and one BJT. In the latter case, the BJT must be the upper transistor; otherwise, the (lower) BJT will always saturate (unless extraordinary steps are taken to bias it).

Advantages

The cascode arrangement offers high gain, high slew rate, high stability, and high input impedance. The parts count is very low for a two-transistor circuit.

Disadvantages

The cascode circuit requires two transistors and requires a relatively high supply voltage. For the two-FET cascode, both transistors must be biased with ample VDS in operation, imposing a lower limit on the supply voltage.

Dual-gate version

A dual-gate MOSFET often functions as a "one-transistor" cascode. Common in the front ends of sensitive VHF receivers, a dual-gate MOSFET is operated as a common-source amplifier with the primary gate (usually designated "gate 1" by MOSFET manufacturers) connected to the input and the 2nd gate grounded (bypassed). Internally, there is one channel covered by the two adjacent gates; therefore, the resulting circuit is electrically a cascode composed of two FETs, the common lower-drain-to-upper-source connection merely being that portion of the single channel that lies physically adjacent to the border between the two gates.

Advantages of modern BJTs (bipolar junction transistors) over GaAs-FETs for low noise preamplifier applications on 3.4GHz.

Traditionally GaAs FETs have been used in the amateur radio community to build very low noise preamplifiers (LNAs) for frequencies in the 100MHz up to many GHz range. In order to achieve extremely low noise figures, it is still common practice to use relatively highly priced devices, that work up to much higher frequencies than what would be needed for a particular design.

While this has enabled the design of LNAs with noise figures of only 1dB and below, the designer always faces significant stability problems when using GaAs FETs for the LNA. This is because theses FETs are able to oscillate at frequencies were bypassing and grounding becomes a challenge of its own. Also lumped elements like chip capacitors are well above their resonance frequency at the frequency of potential oscillation.
Another problem with using these extremely-high-frequency FETs is the very poor input and output match at the frequency of operation. That means the input, as well as the output, must be aggressively matched in order to get them close to 50Ohms. To make things even worse, the input impedance that gives the lowest possible noise figure is usually well away from 50Ohms, so if the input is tuned for minimum noise, the input match of the LNA circuit will be quite poor.
One more inconvenience with GaAs FETs used for LNAs is that most of them require a negative bias voltage, which is usually not available. So additional effort for implementing an voltage inverter must be spend.

While commercial manufacturers of RF communication equipment have widely switched to bipolar transistors for LNA applications, it seems that the amateur radio community has not yet followed this trend so far. Obviously there is a tendency to think that designers of commercial equipment mainly aim for low cost devices and don't care so much about the utmost performance. This may be true to a certain extend, but it does by no means indicate that bipolar transistors would not be an option even for demanding applications. While low cost is an absolute must for commercial designers today, it is also no disadvantage for the radio amateur if the components for homebrew projects are inexpensive. That becomes even more true if a number of devises is damaged during the design phase.

Lets have a look on the Infineon SiGe bipolar transistor BFP620 to understand what the advantages are. First of all it's cheap. Even at single pieces, purchased from an electronics mail order supplier, it is only 0.58 euro cents. And since it is less endangered by static discharge during handling than FETs are, fewer of them will probably be lost during construction. Secondly the BFP620 (as most of the other BJTs) is relatively well matched already. Over the entire frequency range from 100MHz to 6GHz the transducer gain (the devises gain within a 50Ohm environment without matching) is only 1-2 dB less than the theoretically available or the theoretically stable gain with perfect matching. This basically eliminates the need for matching networks. Compared to GaAs FETs, this is very convenient, as these have little to no gain at all, unless they are aggressively matched. Last, but not least important, the impedance for minimum noise figure of the BFP620 is very close to 50Ohm for frequencies from 1.5GHz-5GHz. That means that a preamplifier for 3.4GHz that is build using the BFP620 without any matching at the input, will achieve a noise figure only 0.1-0.2dB above the theoretical optimum. Since it is very likely that at least 0.1-0.2dB would be lost in the matching network due to ohmic losses, it is practically the best solution not to try any kind of input matching and connect the transistor just via a DC blocking capacitor to the antenna. Practically this would result in a noise figure of around 1.3dB for 3.4GHz. This may not be method of choice for amateur radio EME (earth-moon-earth) applications, but for other applications it should really be fine. One should keep in mind that every tenth of a dB of cable loss between antenna and LNA adds directly to the noise figure. So everyone that has a few meters of cable between antenna and LNA should stop thinking about 0.1 or 0.2 dB of noise figure and mount the LNA directly to the antenna first.
It should also be noted that with the BFP620, a noise figure of around 1dB is achieved not only at frequencies much below the intended frequency range of the transistor, but right inside the specified frequency span. This means there is no need to use devices that have an extremely high transition frequency, just to achieve a good noise figure. If we consider the BFP620 again, this device it is already unconditionally stable at frequencies above 5GHz. So oscillation at 10-20GHz is not a problem like it is for advanced GaAs FETs.

Of course where there is so much light, there is also some shadow. Due to its good match, even at low frequencies, the transducer gain peaks at more than 40dB at very low frequencies. With FETs on the other hand, it is possible to put the highest amplification at (or at least close to) the frequency of interest. Only at the frequencies were the (usually narrow banded) matching allows the signal to enter and leave the active device, the circuit will exhibit amplification. Excessive unwanted amplification at lower frequencies may or may not be a problem. If there was only the wanted signal in the air, one would probably not care about more amplification at other frequencies. Unfortunately there are many strong signals in the air today, like FM radio broadcast stations, TV broadcast stations, cell phone base stations etc. All these signals can and will produce intermodulation products within the receiver. The higher the amplification is for these signals, the more trouble they will cause in the receiver.

So, what is necessary to make a nice LNA for 3.4GHz out of a BFP620? First of all one has to make sure it does not oscillate. As mentioned this is a lot easier than for GaAs FETs. But if a high-Q narrow-band filter shall be put after the LNA, the transistor will see total reflection at all frequencies other than the pass band of the filter. This is always a very critical situation in terms of instability. One could place a resistive attenuator between the LNA and the filter, but this would cost some of the gain that the LNA has produced just before. Other than GaAs FETs, the bipolar transistor provides the LNA designer with a nice possibility to solve this problem. Since the BFP620 is prone to instability especially at lower frequency where it exhibits a very high gain, it is very helpful to put a diplexer after the LNA that terminates everything below 2GHz into 50Ohms. This solution has the very nice side effect that it conquers the excessive gain at low frequencies and produces a rather flat frequency response. Simulations show that only by means of that diplexer and a very mild high pass filtering at the input, unconditional stability can be reached [1]. In order to minimize the number of external components, the diplexer at the output can be implemented as part of the bias network that is necessary anyhow.

At the input, a simple coupling capacitor to the antenna could be used, as there is no need for a matching network. In order to reduce intermodulation with the LNA and in order to gain the last bit of stability, it is sensible to put a band pass filter before the input. As every loss before the LNA adds directly to the noise figure, only filter topologies with extremely little loss are acceptable here. Since there is a need for feeding in the DC bias to the base of the transistor anyhow, a LC parallel configuration can be used that provides at least some amount of attenuation at outband frequencies, while causing almost now loss on the wanted signal at 3.4GHz.

How a BJT works

Bipolar junction transistor (BJT) was the first solid state amplifier, which boosted up the solid state electronics revolution. It is a type of a transistor that is used as an amplifying or switching device. They are called bipolar since there are two types of charge carriers, electrons and holes.

A bipolar junction transistor constitutes PN junction diodes that are connected back-to-back and regulate the amount of current that can pass through. A smaller controlling current regulates the flow.

A bipolar junction transistor has three terminals namely collector, emitter and base. The middle layer is called base and the outer layers are called collector and emitter. The standard combination of P-N-P or N-P-N type is used. The current that is to be controlled (main current) flows from the collector to the emitter (or the other way). The controlling current (base current) flows from the base to the emitter (or the other way).

Specifications for a BJT

All the electronics/electrical components are susceptible to damages when the specified voltage or current ratings are overridden. Some of the factors to consider when selecting a BJT are listed below.

§ Power dissipation: The power dissipated by a transistor is equal to the product (multiplication) of collector current and collector-emitter voltage. It is advisable to have a BJT with less power dissipation.

§ Reverse voltages: Maximum permissible reverse bias voltages across the PN junctions are specified for BJTs. This rating is of particular importance when using a bipolar transistor as a switch.

§ Collector current: Every BJT will be supplied with a maximum collector current. This parameter should be considered as it greatly affects the power dissipation rating of the transistor.

§ Saturation voltages: Ideally, a saturated transistor acts as a closed switch contact between collector and emitter, dropping zero voltage at full collector current.

§ Beta: It is a fundamental parameter that defines the amplifying capacity of the bipolar junction transistor. Beta is the ratio of collector current to base current and is highest for medium collector currents, decreasing for very low and very high collector currents.

BJT applications

Bipolar junction transistors remain important devices for ultra-high-speed discrete logic circuits such as emitter coupled logic (ECL), power-switching applications and in microwave power amplifiers. BJTs are universally used in electrical circuits where current needs to be controlled. Some of the areas are: switching elements to control DC power to a load, amplifiers for analog signals, 3D bipolar simulation, NPN device, AC frequency response, emitter-coupled logic element simulation, 3-phase AC motors.

Common emitter Common base Common collector

Darlington

Schottky Transistors

When the input of a saturated transistor is changed, the output does not change immediately; it takes extra time, called storage time, to come out of saturation. In fact, storage time accounts for a significant portion of the propagation delay in the original TTL logic family.

Storage time can be eliminated and propagation delay can be reduced by ensuring that transistors do not saturate in normal operation. Contemporary TTL logic families do this by placing a Schottky diode between the base and collector of each transistor that might saturate, as shown in Figure BJT-7. The resulting transistors, which do not saturate, are called Schottky-clamped transistors or Schottky transistors for short.

When forward biased, a Schottky diode’s voltage drop is much less than a standard diode’s, 0.25 V vs. 0.6 V. In a standard saturated transistor, the base-tocollector voltage is 0.4 V, as shown in Figure BJT-8(a). In a Schottky transistor, the Schottky diode shunts current from the base into the collector before the transistor goes into saturation, as shown in (b). Figure BJT-9 is the circuit diagram of a simple inverter using a Schottky transistor.

Transistor Logic Inverter

Figure BJT-4 shows that we can make a logic inverter from an npn transistor in the common-emitter configuration. When the input voltage is LOW, the output voltage is HIGH, and vice versa.

In digital switching applications, bipolar transistors are often operated so they are always either cut off or saturated. That is, digital circuits such as the inverter in Figure BJT-4 are designed so that their transistors are always (well, almost always) in one of the states depicted in Figure BJT-5. When the input voltage VIN is LOW, it is low enough that Ib is zero and the transistor is cut off; the collector-emitter junction looks like an open circuit. When VIN is HIGH, it is high enough (and R1 is low enough and is high enough) that the transistor will be saturated for any reasonable value of R2; the collector-emitter junction looks almost like a short circuit. Input voltages in the undefined region between LOW and HIGH are not normally encountered, except during transitions. This undefined region corresponds to the noise margin that we discussed.

Another way to visualize the operation of a transistor inverter is shown in Figure BJT-6. When VIN is HIGH, the transistor switch is closed, and the output terminal is connected to ground, definitely a LOW voltage. When VIN is LOW, the transistor switch is open and the output terminal is pulled to 5 V through a resistor; the output voltage is HIGH unless the output terminal is too heavily loaded (i.e., improperly connected through a low impedance to ground).

BJT.1 Basic Operation

A bipolar junction transistor is a three-terminal device that, in most logic circuits, acts like a current-controlled switch. If we put a small current into one of the terminals, called the base, then the switch is “on”—current may flow between the other two terminals, called the emitter and the collector. If no current is put into the base, then the switch is “off”—no current flows between he emitter and the collector.

To study the operation of a transistor, we first consider the operation of a pair of diodes connected as shown in Figure BJT-1(a). In this circuit, current can flow from node B to node C or node E, when the appropriate diode is forward biased. However, no current can flow from C to E, or vice versa, since for any choice of voltages on nodes B, C, and E, one or both diodes will be reverse biased. The pn junctions of the two diodes in this circuit are shown in (b). Now suppose that we fabricate the back-to-back diodes so that they share a common p-type region, as shown in Figure BJT-1(c). The resulting structure is called an npn transistor and has an amazing property. (At least, the physicists working on transistors back in the 1950s thought it was amazing!) If we put current across the base-to-emitter pn junction, then current is also enabled to flow across the collector-to-base np junction (which is normally impossible) and
from there to the emitter.

The circuit symbol for the npn transistor is shown in Figure BJT-1(d).
Notice that the symbol contains a subtle arrow in the direction of positive current flow. This also reminds us that the base-to-emitter junction is a pn junction, the same as a diode whose symbol has an arrow pointing in the same direction.

It is also possible to fabricate a pnp transistor, as shown in Figure BJT-2.
However, pnp transistors are seldom used in digital circuits, so we won’t discuss them any further.

The current Ie flowing out of the emitter of an npn transistor is the sum of the currents Ib and Ic flowing into the base and the collector. A transistor is often used as a signal amplifier, because over a certain operating range (the active region) the collector current is equal to a fixed constant times the base current.

However, in digital circuits, we normally use a transistor as a simple switch that’s always fully “on” or fully “off,” as explained next. Figure BJT-3 shows the common-emitter configuration of an npn transistor, which is most often used in digital switching applications. This configuration uses two discrete resistors, R1 and R2, in addition to a single npn transistor. In this circuit, if VIN is 0 or negative, then the base-to-emitter diode junction is reverse biased, and no base current (Ib) can flow. If no base current flows, then no collector current (Ic) can flow, and the transistor is said to be cut off (OFF).

BJT vs MOSFET

The transistors BJT and MOSFET are both useful for amplification and switching applications. Yet, they have significantly different characteristics.

BJT, as in Bipolar Junction Transistor, is a semiconductor device that replaced the vacuum tubes of the old days. The contraption is a current-controlled device where the collector or emitter output is a function of the current in the base. Basically, the mode of operation of a BJT transistor is driven by the current at the base. The three terminals of a BJT transistor are called the Emitter, Collector and Base.

A BJT is actually a piece of silicon with three regions. There are two junctions in them where each region is named differently – the P and N. There two type of BJTs, the NPN transistor and the PNP transistor. The types differ in their charge carriers, wherein, NPN has holes as its primary carrier, while PNP has electrons.

The operation principles of the two BJT transistors, PNP and NPN, are practically identical; the only difference is in biasing, and the polarity of the power supply for each type. Many prefer BJTs for low current applications, like for switching purposes for instance, simply because they’re cheaper.

Metal Oxide Semiconductor Field-Effect Transistor, or simply MOSFET, and sometimes MOS transistor, is a voltage-controlled device. Unlike the BJT, there is no base current present. However, there’s a field produced by a voltage on the gate. This allows a flow of current between the source and the drain. This current flow may be pinched-off, or opened, by the voltage on the gate.

In this transistor, a voltage on an oxide-insulated gate electrode can generate a channel for conduction between the other contacts – the source and drain. What’s great about MOSFETs is that they handle power more efficiently. MOSFETs, nowadays, are the most common transistor used in digital and analog circuits, replacing the then very popular BJTs.

Summary:
1. BJT is a Bipolar Junction Transistor, while MOSFET is a Metal Oxide Semiconductor Field-Effect Transistor.
2. A BJT has an emitter, collector and base, while a MOSFET has a gate, source and drain.
3. BJTs are preferred for low current applications, while MOSFETs are for high power functions.
4. In digital and analog circuits, MOSFETs are considered to be more commonly used than BJTs these days.
5. The operation of MOSFET depends on the voltage at the oxide-insulated gate electrode, while the operation of BJT is dependent on the current at the base

About Small-signal Bipolar Transistors (BJT)

Small-signal bipolar transistors (BJT) are semiconductors that amplify small AC or DC signals. They consist of a base n-type or p-type layer sandwiched between emitter and collector layers of the opposite type. With small-signal bipolar transistors, there are two polarities available: PNP and NPN. PNP devices consist of an n-type layer sandwiched between two p-type layers. NPN devices consist of a p-type layer sandwiched between two n-type layers. In both arrangements, the junctions between layers amplify weak incoming signals and the current flow between the emitter and the base controls the current flow between the emitter and the collector. Based on this design, even a small current between the base and emitter connections results in a large current between the emitter and collector connections. In circuits that use small-signal bipolar transistors as switching devices, current in the base-emitter junction creates a low-resistance path between the collector and the emitter.

Performance specifications for small-signal bipolar transistors (BJT) include collector-to-emitter breakdown voltage, collector-to-base breakdown voltage, maximum collector current, current gain bandwidth, and temperature range. Static forward current transfer ratio, which is also known as common-emitter current gain, is the ratio of the input DC current and the output DC current. Power dissipation, the total power consumption of the device, is usually measured in watts (W) or milliwatts (mW). Temperature range for small-signal bipolar transistors is measured in degrees Fahrenheit or degrees Celsius.

Basic IC package types for small-signal bipolar transistors are transistor outline (TO), small outline (SO), and small outline transistor (SOT). For each package type, many variants are available. Transistor outline packages include TO-92, a single in-line package often used for low power devices; TO-220, which is suitable for high power, medium current, and fast-switching power devices; and TO-263, the surface-mount version of the TO-220 package. Small outline transistor packages include SOT23, which is often used in home appliances, office and industrial equipment, personal computers, printers, and communication equipment; SOT89, a plastic, surface mounted package with three leads and a collector pad for good heat transfer; and SOT223, an encapsulated package that provides excellent performance in environments with high temperatures and humidity levels. IC package types for small-signal bipolar transistors include discrete or deca-watt package (DPAK) and flat package (FPAK).

Packing methods for small-signal bipolar transistors (BJT) consist of tape reel, rail, bulk pack, and tube technologies. The tape reel method packs components in a tape system by reeling specified lengths or quantities for shipping, handling, and configuration in industry-standard automated board-assembly equipment. Rail, another standard packing method, is typically used only in production environments. Bulk pack devices are distributed as individual parts, while tray components are shipped in trays. The tube or stick magazine method is used to feed small-signal bipolar transistors into automatic placement machines for through-hole or surface mounting.

Transistor Physics

• Composed of N and P-type Semiconductors

• N-type Semiconductor has an excess of
electrons
– Doped with impurity with more valence electrons
than silicon

• P-type Semiconductor has a deficit of
electrons (Holes)
– Doped with impurity with less valence electrons
than silicon

P-N Junction (Basic diode):

- Bringing P and N Semiconductors in contact
- Creation of a Depletion Zone

P-Type N-Type

• P-N Junction
• Reverse Biased => No Current
• Applying –ve Voltage to Anode increases
Barrier Voltage & Inhibits Current Flow

• P-N Junction
• Forward Biased => Current Flows
• Applying +ve Voltage > Barrier Voltage to
Anode allows current flow


• Basic Transistor


Water pipe analogy






Properties of the BJT

Common emitter configuration

2 basic laws:

Ie=Ib+Ic
Ic=β.Ib (β=10 to 100)


Operating Point

• Amplifier mode
• Switching mode

Silicon bipolar junction transistors (BJTs)

Silicon bipolar junction transistors (BJTs) technology has been a dominant RF power device in solid-state pulsed amplifiers up to L-band frequencies for many years. They can currently achieve at least 1-kW of peak RF power in push-pull operation. Of the technologies that are potential alternatives to silicon BJTs in this application – VDMOS, GaAs, SiC, LDMOS, and most recently GaN, only LDMOS has thus far proven competitive. That is, LDMOS can deliver the required RF output power, high efficiency, gain, and linearity, along with ruggedness over wide-ranging operational conditions. As a result, LDMOS FETs are making significant headway in replacing BJTs in solid state amplifiers operating in Class C mode at frequencies up to 1400 MHz. A comparison of the two technologies (Table 1) shows the strengths of each. Two of the 50-V LDMOS devices, for 450 and 1200 to 1400 MHz, designed and manufactured by Freescale show the performance LDMOS can achieve for pulsed applications.



Freescale has for several years been working on increasing the frequency range of the 50V technology, and has produced a family of devices for operation at frequencies from UHF through L-band. The first example is the MRF6VP14300H, which delivers 330 W from 1200 to 1400 MHz with a 300 µs pulse width signal and 12% duty cycle at 150 mA quiescent current bias. It was the first reported LDMOS device to deliver this performance at this frequency. Power density is 1.89 W/mm, drain efficiency is more than 59%, and gain is 14 dB in Class C operation. Rise time is less than 60 ns and pulse droop only 0.4 dB. Thermal resistance is 0.13ºC/W and junction temperature remains below 100º C at a flange temperature of 65º C. Power gain and efficiency versus output power at 1.4 GHz is shown in Figure 1.

The MRF6VP14300H employs internal matching for both the input and output side of the transistor to transform the die level impedance, which makes it easy for the designer to match the transistor input and output impedances to 50 ohms across the 1.2 to 1.4 GHz band. The internal input match consists of a two-section, lowpass T-network, and the internal output match consists of a single shunt inductor in series with a capacitor, which can improve output bandwidth.

The device can handle 3-dB input power overdrive and can withstand a 5:1 VSWR mismatch with full phase variation under a 3-dB overdrive condition. With the built-in enhanced ESD protection circuit, the device can operate in Class C with 14 dB of gain. The gate threshold voltage is typically around 1.6 V. Figure 2 shows the test results of Vgs sweep from 1.11 V to 2.23 V, and the operation class shifts from C to B to AB. Input power ranges up to 15 W, which is 3 dB higher than the typical maximum input power of 7.5 W. Power gain in Class C is 3 dB lower than for Class AB although power gain and output power nearly converge when the input power is greater than 12 W.

Figure 3 shows the gain and output power at Vgs of 0.84 V to 1.11 V and 12% duty cycle. Quiescent current is 3 mA. The device is operated in Class C at different Vgs and gain is close at higher input powers. However, the device provides isolation when input power is lower than 500 mW at Vgs of 0.84 V, which is very important for the stability of the host system. The pulse waveform of a pulsed transmitter is its core “ingredient’, so slow transistor rise time and power droop within the pulse will produce a commensurate droop at the transmitter output, with a serious negative effect on system performance. This is important as well because many transistors are combined to produce the final output. Rise time of the MRF6VP14300H is less than 60 ns and pulse droop less than 0.4 dB from 1.2 to 1.4 GHz.

1 kW Performance at 450 MHzThe MRF6VP41KH is another example of Freescale’s 50-V capabilities, this time at 450 MHz. It can deliver 1 kW peak RF output with a 300-us pulse width (20% duty cycle) input signal. Efficiency is greater than 60% and gain is 17 dB at 450 MHz in Class C operation. It will handle a VSWR of 5:1 with 300 and 500 µs pulses at a 20% duty cycle. In fact, this device can be used in CW applications, where it can deliver 1kW at 352MHz.

The MRF6VP41KH is designed in a push-pull configuration. There is no internal-matching capacitor at the input and output (device output capacitance is less than 150 pF), so that an external matching network can be used to achieve high performance over a wide bandwidth. While LDMOS FETs have been very successful in wireless infrastructure applications in Class AB mode (where high linearity is critical), their performance in class C is also quite remarkable.An on-chip circuit that provides considerable protection from electrostatic discharge (ESD) makes the device less susceptible to stray voltage during design and production. It provides the additional benefit of accepting a wide range of gate voltages from -6 to +10 VDC, which improves its performance when operates in Class C mode. The device has been built to accommodate both flanged (Figure 4) and earless packages to suite specific applications.

A comparison of the measured performance of the LDMOS device in Class AB, Class B, and Class C operation is shown in Figure 5. Output power and power gain versus input power are shown for a 300-µs pulse width and 20% duty cycle at 450 MHz under different Vgs conditions, effectively shifting operation from Class C to Class B. The device shows good linearity before the P1dB point and power gain remains near constant over a 3-dB range of input power (10 to 20 W). Power gain converges after input power increases to more than 22 W. Figure 6 illustrates efficiency at different Vgs bias conditions and is greater than 60% with input power greater than 18 W at 450 MHz. The droop performance at 450 MHz of the MRF6VP41KH is less than 0.3 dB at room temperature when delivering 1 kW of RF power. Rise time is less than 500 ns and fall time is less than 100 ns.

SummaryThe silicon BJTs has earned its solid position in lower-frequency pulsed power amplifiers (and other applications as well) through its ability to deliver high power with very good performance in nearly all the parameters that matter most. However, LDMOS devices are encroaching on the territory that is currently the BJT’s near-exclusive domain, and will increase their penetration of this market because of their superior long-term “roadmap” to greater performance and higher frequencies as well as competitive specifications in nearly every area. The two devices described in this article are currently in production at Freescale, and more information is available at our website.

INTRODUCTION TO
TRANSISTORS


What is a Transistor?

A Transistor is an electronic device
composed of layers of a semiconductor
material which regulates current or
voltage flow and acts as a switch or gate
for electronic circuit.


History of the Transistor

P-N Junction
Russell Ohl 1939

First Transistor
Bell Labs 1947

Shockley, Brattain,
and Bardeen

First Solid State
Transistor – 1951





Processor development followed Moore’s Law

1965 30 Transistors

1971 15,000

2000 42 million

2x growth every 2 years

Applications

• Switching

• Amplification

• Oscillating Circuits

• Sensors