Download THE CASCODE AMPLIFIER: A common-gate (common

THE CASCODE AMPLIFIER:
A common-gate (common-base) amplifier stage in cascade with a common source (commonemitter) amplifier stage, results in a very useful and versatile amplifier circuit known as the
cascode configuration and has been in use in a wide variety of technologies for over three
quarters of century.
The basic idea behind the cascode amplifier is to combine the high input resistance and large
transconductance achieved in a common-source (common-emitter) amplifier with the currentbuffering property and the superior high-frequency response of the common gate (common-base)
circuit. The cascode amplifier can be designed to obtain a wider bandwidth but equal dc gain as
compared to the common-source (common-emitter) amplifier. Alternatively, it can be designed
to increase the dc gain while leaving the gain bandwidth product unchanged.
In many applications the cascade amplifier is thought of and treated as a single-stage amplifier
though it is formed by cascading two amplifier stages.
The MOS Cascode
Figure 20(a) shows the MOS cascode amplifier. Here transistor Q1 is connected in the commonsource configuration and provides its output to the input terminal (i.e., source) of transistor Q2.
Transistor Q2 has a constant dc voltage, VB1AS, applied to its gate. Thus the signal voltage at
the gate of Q2 is zero, and Q2 is operating as a CG amplifier with a constant-current load, I.
Obviously both Q1 and Q2 will be operating at DC drain currents equal to I. As in previous
cases, feedback in the overall circuit that incorporates the cascode amplifier establishes an
appropriate dc voltage at the gate of Q1 so that its drain current is equal to I. Also, the value of
VBIAS has to be chosen so that both Q1 and Q2 operate in the saturation region at all times.
Small-Signal Analysis : In response to the input signal voltage vi the common-source transistor
Q1 conducts a current signal gm1vi, in its drain terminal and feeds it to the source terminal of the
common-gate transistor Q2, called the cascode transistor.
Transistor Q2 passes the signal current gm1vi on to its drain, where it is supplied to a load
resistance RL (not shown in fig. 20) at a very high output resistance, Rout. The cascode transistor
Q2 acts in effect as a buffer, presenting a low input resistance to the drain of Q1 and providing a
high resistance at the amplifier output.
Characteristic parameters: In Fig. 20(b) shows the cascode circuit prepared for small signal
analysis and with a resistance RL shown at the output. RL is assumed to include the output
resistance of current source I as well as an actual load resistance, if any. The diagram also
indicates various input and output resistances obtained using the results of the analysis of the CS
and CG amplifiers in previous sections. Note in particular that the CS transistor Q1 provides the
cascode amplifier with an infinite input resistance. Also, at the drain of Q1 looking "downward”,
see the output resistance of the CS transistor Q1, ro1. Looking "upward," we see the input
resistance of the CG transistor Q2,
Assume all MOSFETs have W/L of 7.2 μm/0.36 μm and are operating at ID = 100 μA, gm
= 1.25 mA/V, χ = 0.2, r0 = 20 kW, Cgs = 20 fF, Cgd = 5fF,Cdb= 5 fF, and CL (excluding
Cdb= 5 fF. For case (a), let RL= r0= 20 kW for both amplifiers. For case (b), let RL =
r0 = 20 kW for the CS amplifier and RL = Rout for the cascode amplifier. For all cases,
determine Av, fH and ft.