Ing Electronica: Frequency Response for MOSFET/BJT

The frequency response of a BJT or MOSFET can be found using nearly the exact same process, with the only variations being caused by a single resistor and simple naming conventions that differ between the two devices.

Before we start let’s think a little bit about what we’re doing:
Our goal is going to be to find the pole(s) of the circuit.
Okay? What is a pole and why do I care where it is?
A pole is a frequency at which the gain of the device rolls off. (remember that when it rolls off , it will be at the -3dB frequency with a slope of -20dB/decade)

We care because if the gain of a device rolls off at a certain frequency, then we won’t be able to amplify a signal above that frequency very well because the gain will be decreasing by 20dB/decade.

The procedure is nearly identical whether we are using a BJT of a MOSFET, but we will work each of them side by side just in case there might be any confusion, and we’ll follow these steps as we go through. (we will also use some values that came from the output file when running a simulation of this circuit in Cadence (or PSPICE) ) MOSFET

BJT
1. Take a look at one of the circuits and see what you notice, how about the MOSFET. This step is just to help us with our knowledge understanding of the circuit.
- At a glance it just looks just like another MOSFET right? Sure is, but let’s take a look at a few things just for kicks. Notice that it is using a bypass capacitor at the source so we don’t have to worry about $R_s$ (at when working with high frequency). Since the capacitor $C_s$ bypasses $R_s$ to ground, you should notice that this is a common-source amplifier. You could notice the Values for $R_1$ and $R_2$ and start to think about what the Gate voltage is and how that may affect the circuit.
2. We are talking about frequency response so that means we are probably going to want to draw the small signal equivalent circuit.
Remember that the capacitors $C_1$ and $C_2$ will act like short circuits at high frequencies so we will ignore them, but we will have to account for some of the capacitance internal to the device.

Both devices have internal capacitances that are very similar. As you can see from the small signal models for a MOSFET (above) and BJT (below), the only significant difference is that the BJT has an additional resistance Rpi between the Base and Emitter.

Most of the analysis we will do is based on the small signal model. Note that small signal models are not typically used in PSPICE so this picture may look a bit odd, especially the controlled source but for our purpose it is good to have a visual reference. To start we will point out what everything is. Cgs is an internal capacitance betwe

en the gate and source. The

values for Cgs was similar to one the a PSPICE simulation may give. CM1 and CM2 are Miller capacitances which we will find values for later. ro is a Norton equivalent resistance that makes the model more ideal. And just pretend that the G2 looks more like a voltage controlled current source and that their gains are gm*Vgs and gm*Vpi. For the BJT CM1 and CM2 are both Miller capacitances, Cpi is similar to Cgs and Rpi the additional component used for BJTs but not MOSFETs. The other part should look familiar from the other figures.

ON TO THE ANALYSIS!!!

We will find the device gain, overall gain, equivalent input and output capacitances, and the input and output poles. The process for both is essentially the same.

Device Gain: This is the gain from the control source to the output so we are looking for Vout/Vgs (or Vout/Vpi for a BJT). We will ignore CM2 for this process. Notice the resistances ro, RD, and RL are in parallel. Vout should be given by that equivalent resistance times the current though it which is gm*Vgs from the control source. So the equation for device gain is,

$V_{out} / V_{gs} = gm*(r_o//R_D//R_L)$ (MOSFET)

$V_{out} / V_{\pi} = gm*(r_o//R_C//R_L)$ (BJT)

Overall Gain: This will be the gain from the source (Vs) to the output (Vout). We already know what Vout/Vgs is so if we find Vgs/Vs, we can multiply them to get Vout/Vs = (Vout/Vgs) * (Vgs/Vs). Vgs/Vs is a simple voltage divider. Hopefully you can see this from the small signal model (remember that we are ignoring the capacitors for now but they will play a part later). The equations we will get for Vgs/Vs and the overall gain are.

$V_{gs} / V_s = \frac{ (R_1//R_2)}{(R_1//R_2) + R_s}$ (MOSFET)

Overall Gain: $V_{out} / V_s = \frac{ (R_1//R_2)}{(R_1//R_2) + R_s} * gm(r_o//R_D//R_L)$ (MOSFET)

$V_{gs} / V_s = \frac{ (R_1//R_2//r_\pi)}{(R_1//R_2//r_\pi) + R_s}$ (BJT)

Overall Gain: $V_{out} / V_s = \frac{ (R_1//R_2//r_\pi)}{(R_1//R_2//r_\pi) + R_s} * gm(r_o//R_C//R_L)$ (BJT)

Now we will find the input and output poles. For this we will need to look at the capacitances and use a formula to find the Miller capacitances, CM1 and CM2. Any explanation for the miller capacitance will have to wait for another post or check out your Electronics Book, Wikipedia, Google, etc. but we will need to use a couple of special equations. Overall we will need to find the input resistance and input capacitance for the input pole and the output resistance and output capacitance for the output pole.

Each pole will be at a frequency w=1/RC where the R and C are the equivalent R and C at that point, so to find the input pole, we will need to find the input resistance and the input capacitance. These are found by looking into the input (the left side of the small signal model). The voltage source will act like a short so we see Rs in parallel with R1//R2 for the MOSFET (the BJT will have Rpi in parallel also). The input capacitance will be Cgs in parallel with CM1 (the BJT will be the same). The output resistance and capacitance are found the same way only looking in from the output (the right side of the small signal model).

$\omega_{IN} = \frac{1}{R_{IN}C_{IN}}$ $\omega_{OUT} = \frac{1}{R_{OUT}C_{OUT}}$ (MOSFET or BJT)