Useful Circuits with Two Transistors

The preeminent circuit designer Barrie Gilbert asks "How many distinctly different and really useful circuits can be made with two transistors, anyway?"  and his answer "about twenty-four" (Williams, Analog Circuit Design, p179).  Let's see what they might be.  It is perhaps subjective to tell what is useful or different.  Also what type transistors, BJT, JFET, MOSFET?  Same circuit configuration with different type of transistors should not be considered distinct. What about other circuit elements?  Assume the passives, resistors, capacitors, inductors, are OK, but what about diodes? And what about a multi-emitter/collector transistor or a dual-gate MOSFET?  Is it considered one transistor or multiple transistors?

A single transistor has three basic amplifier configurations; it can also be configured as a diode, and the emitter-base junction also makes a somewhat usable Zener diode.  A JFET makes a good current source (current regulating diode).

Input Impedance of a Bipolar Transistor Bias with Feedback

AoE shows a bipolar transistor bias circuit using feedback from the collector.  AoE2 and AoE3 show the identical circuit, but they differ in the values of the input impedance: AoE2 states 300 Ohms and AoE3 200 Ohms.  We would like to analyze the circuit to see why the value has changed.  Here the base bias is established by a resistor from the collector, which has the effect of negative feedback: a high bias lowers the collector voltage that reduces the base voltage.  

Here we estimate the collector current to be about 1mA.  The input impedance is about 200 Ohms for a beta of 100; it does not exceed 225 Ohms as the beta goes to infinity.  So it seems justified that AoE3 makes the correction.   Even when the temperature varies, the input resistance stays relative constant.  For example, if T = 100C, VT = kT/q = 32mV, but because Vbe decreases with increasing temperature, the emitter current also goes up, Vbe = 0.4V, Ie = 1.34mA, gm is staill around 1/25 A/V.

N Light Switches

AoE2 has an exercise asking readers to wire up N switches that any one of the switches can turn on or off a light bulb.   It comments that every electrician knows how to do this, but few electronic circuit designers do.  Given the hint that it requires 2 SPDT and N-2 DPDT switches, it is not hard to do.

This exercise is retained in AoE3.

A Current Source Circuit from AoE-X

H&H show a clever current source circuit and challenge readers to prove the circuit works especially without using two of the constraints.

It is a circuit of an opamp driving a bipolar transistor.  The base current of the bipolar transistor would normally cause an error.  Here the error is compensated.  The base current is sensed with the base resistor kR1; the emitter voltage is Vin with the differential voltage added using a circuit configuration resembling a difference amplifier.

The derivation makes no use of the constraints on the resistors.  The base current compensation is strictly accurate (assuming ideal opamp).