CMOS Common Gate Amplifier.
The IC version of the common gate amplifier with an active load is shown below implemented in CMOS:
The common gate amplifier functions similar to a BJT common base amplifier as we discovered in Lecture 34. In the CMOS implementation of this amplifier shown above, Q2 and Q3 function as a current mirror (as with the CMOS common source amplifier we studied in Lecture 33) with Q2 further serving as an active load.
There is one subtlety with this IC amplifier that can affect its performance that’s not found in the other two types of CMOS amplifiers. It’s called the body effect. We’ll first study this MOSFET body effect and then come back to the analysis of the CMOS CG amplifier.
MOSFET Body Effect
In ICs, the body or substrate is shared amongst all (or most) of the transistors.
To guarantee that the body-to-source junction (and the other parts of the channel) remain reversed biased for all transistors in the IC, the substrate is connected to -Vss , which is the smallest voltage in the circuit:
The reason is that if any body-to-source pn junctions (or any other parts of the channel) become forward biased, there would be a catastrophic failure in the transistor operation!
To avoid this problem, one idea is to just connect the body terminal to the source terminal for all transistors in the IC. For the common gate amplifier, however, we see there is a problem in doing this. For circuits with input at the source (such as the common gate amplifier), we don’t want the source connected to the most negative voltage in the circuit (or whatever voltage is assigned to the body).
So instead, we’ll connect VB to -VSS. The question now is what affect will this have on the device operation? Referring to Fig. 1, the resulting reverse-bias voltage between the source and body (VSB for NMOS):
1. Will widen the depletion region.
2. This, in turn, will decrease the channel depth. (To return the channel to its original depth, vGS would need to be increased.) It can be shown that this effect of VSB on the channel can be modeled as simply a change in the threshold voltage according to
• Vt0 is the threshold voltage for V SB=0
• 2 jf is the material dependent Fermi potential (often 2 jf» 0.6 V for NMOS)
• g is a fabrication process parameter (the body-effect parameter) given as
(often g 0.4 FONT FACE="Symbol">» V1/2).
Notice in (1) that with this body effect a change in VSB produces a change in Vt. This change in Vt will change iD, even though vGS doesn’t change. (Why? Because the channel depth changes with changing VSB, as we’ve just learned.) Consequently, this body voltage VSB controls iD like vGS does.In other words, the body is acting like a second gate (of sorts), sometimes called the backgate. This phenomenon is called the body effect. It can degrade the performance of MOSFET amplifiers in some cases.
Small-Signal Modeling of the Body Effect
A change in the body-to-source voltage vBS will cause a change in the drain current iD, as we’ve just seen. This behavior can be quantified for time varying signals by the relationship
id= gmd vbs
where gmb is the so-called body conductance
It can be shown that
where is dimensionless and typically lies in value between 0.1 and 0.3.For small-signal modeling, in light of (3) a second dependent current source is added to the MOSFET equivalent circuit, as shown in Fig 4.41, which is dependent on vbs.
Alternatively, the small-signal T model for the MOSFET amplifier including the body effect is:
Common Gate Amplifier with Active Load
The text models the active load for this CG amplifier as a simple ideal current source as shown in Fig. 6.27(a):
Note here that the body terminal is not connected to the source terminal, but rather is connected to the lowest voltage in the circuit (ground). Because of this we need to account for the body effect in the small-signal T model of this amplifier:
Notice that we’ve incorporated the body effect into the T smallsignal model of the MOSFET. Because the gate and body are both grounded in Fig. 2, then
Consequently, the body effect in this CG amplifier can be completely accounted for by simply replacing gm of the MOSFET with gm + gmb. That is,
Small-Signal Amplifier Characteristics
We’ll calculate the following small-signal quantities for this CMOS common gate amplifier: Rin, Av, Gv, Gi, and Rout.
• Input resistance, Rin. Using KCL at the source terminal in Fig. 2 as well as (6) we find
An important insight into this circuit can be realized by examining the “supernode” contained in the dashed outline in the small-signal circuit. Because 0 g b i i = = , applying KCL to this supernode
Since vgs=- vi and
then (8) becomes
From this result, we can quickly determine that
We see from this expression that the body effect tends to reduce the input resistance. Notice that Rin depends on RL. This type of amplifier is called a bilateral am plifier, in contrast to a unilateral amplifier in which Rin is not dependent on RL.
• Partial small-signal voltage gain, Av. At the output side of the small-signal circuit
while at the input
Dividing these two equations we arrive at the partial smallsignal voltage gain
Here we see that the body effect tends to increase Av.
• Overall small-signal voltage gain, Gv. As we have done many times in the past for other types of amplifiers
Substituting for Av from (15) gives
Since the body effect tends to reduce Rin, we see from this expression that it tends to increase Gv.
• Overall small-signal current gain, Gi. From (9) we see directly that
• Output resistance, Rout. To determine Rout from the smallsignal circuit above we set vsig =0 and apply a fictitious AC voltage source vx at the output as shown:
We can see that with vx attached, the voltage vgs will not usually be zero. This means the current in the dependent current source is also not zero. Consequently, we need to analyze this circuit – including the effects of the dependent current source – to determine the output resistance. Employing KCL at the drain terminal
It is easy to see in Fig. 3 that
Applying KCL to the supernode indicated in Fig. 3, we find
then using this in (20) and substituting into (19)
From the small-signal circuit in Fig. 3
Substituting (24) into (23) gives
We see from this result that the body effect tends to increase Rout. Since Rout depends on Rsig, this is another reason why this amplifier is a bilateral rather than a unilateral amplifier.
Summary of the Small-Signal Characteristics
In summary, we find for the CG small-signal amplifier:
o A non-inverting amplifier [see (17) and (18)].
o Moderately large input resistance. From (12), with r0 =j (10 5) ,Rin = j (103), gm ,gmb- = O (1010-3), then from (12),Rin j=(103)
o Relatively modest small-signal voltage, depending on the choices for RL and Rsig [see (17)].
o Small-signal current gain equal to one [see (18)].
o Potentially very large output resistance [see (25)].
These are characteristics similar to the discrete version of the common gate amplifier we discussed in the previous lecture. No surprise