3 × 3 options

1. Introduction

Single transistor amplifier properties:

  • Bias (DC) conditions to set up IC (yielding the small-signal values of re, gm, rpi, ro)

  • Impedances looking into each transistor terminal.

  • Thevenin equivalent output impedance.

  • Open-circuit (voltage) gain. (amplifier version of Thevenin voltage)

Be precise with the vocabulary:

Series

The same current flows through both things.
Cascade

The output of one feeds the input of the next.
Table 1. Names for two cascaded transistor amplifiers
Second

First

EF

CE

CB

EF

Darlington pair
super beta transistor

high input impedance V-gain stage

¿diff-pair with single-ended input?

CE

gain + V-buffer

(gain)2

cascode

CB

low impedance current input + voltage output
Historical background for the name “casc-ode”

From the “cascaded cathode” amplifier, a common-cathode followed by a common-grid amplifier.

Triode schematic labeled
Figure 1. Vacuum tube triode symbol [1]
Table 2. Congruent terminals
triode BJT FET

plate

collector

drain

grid

base

gate

cathode

emitter

source

2. Darlington pair

240px TIP122
Figure 2. From TIP122 die photo
darlington sym
Figure 3. Darlington pair

TIP122 datasheet

  • In forward-active mode: What are the terminal currents of the composite structure?

3. Sziklai pair

sziklai pair
Figure 4. Sziklai pair

  • In forward-active mode: What are the terminal currents of the composite structure?

4. Cascode: +

cascode 1
Figure 5. Cascode amplifier

If you view this as two, 1-transistor amplifiers, what are their type names?

  • Q1:

  • Q2:

Don’t forget the DC solution! What should the voltage at node A be?[2]




What is the minimum voltage at node X that keeps Q1 functionally in active mode?




What is the minimum voltage at node B, then?  State as an inequality.




In practice, the base terminal of the second stage’s common-base transistor (node B here) is nearly always AC-grounded by a capacitor between the base and a power supply or reference node (node declared to be 0V for the purposes of nodal analysis).

Refer to Tourbook Table 10 for the following:

  • What does the value of ZB for Q2 become in these common cases?



  • The input impedance Zi of the CB stage then collapses to become:

4.1. Small-signal model

Draw the small-signal model of this CE + CB amplifier.

We have been using a Thevenin-equivalent model for the higher-level view of an amplifier (voltage gain and series output impedance). For the CE stage in this structure, it is useful to model in its native form with the controlled current source gm and use (keep) the Norton equivalent form.

4.2. Another circuit to small-signal model

folded cascode
Figure 6. Mystery two-stage amplifier[3]

Draw the small-signal model of this amplifier. Assume that Cin, Cb, and Cout are all BFCs and thus short-circuits at all frequencies of interest. Compare to the last amplifier’s small-signal equivalent circuit.

5. References

For more transistor arrangements, see Fifty Nifty Variations of Two-Transistor Circuits: A tribute to the versatility of MOSFETs in IEEE Solid-State Circuits Magazine, Summer 2021. local copy

1. https://en.wikipedia.org/wiki/Triode#/media/File:Triode_schematic_labeled.svg
2. In this case: must be
3. A folded cascode amplifer.