A complementary-feedback-pair voltage buffer

September 20, 2010 2:20:23 PM CDT

The photos to the right show one of my latest experiments in prototyping.. specifically, seeing if I can build an interesting circuit using my 40-pad protoboards.

As it turns out, the answer is yes.

The circuit in question is a complementary buffer. The two transistors marked in red are 2N3904 NPNs, the blue one is a 2N3906 PNP. Moving from left to right in the top picture, I'll call them Q1 (NPN), Q2 (PNP), and Q3 (NPN).

Q1 (NPN) sits between resistors going to Vcc and GND, and gets an input voltage from the trimpot to the left.

Q2 (PNP) is driven by the voltage at Q1's collector. As the input to Q1 rises, its collector voltage falls, allowing Q2 to come open.

Q3 (NPN)'s base and collector are tied together.. it's what's known as a 'diode connected' transistor because it behaves almost like a perfect diode. Q2's base and collector are tied to Q2's collector, and Q2's emitter is tied to the same resistor as Q1's emitter.

When the voltage from the trimpot rises, Q1's base-emitter voltage increases, allowing it to pass more current.

Current through the resistor going to Vcc causes the voltage at Q1's collector to fall. That brings Q2's base voltage down, allowing it to open and pass current.

Current from Q2 goes straight through Q3 and into the resistor that goes to GND. The additional current from Q2-Q3 causes Q1's emitter voltage to rise, decreasing its base-emitter voltage and reducing the current through Q1.

The upshot is that you get a negative feedback loop that tracks the voltage at Q1's base.

The voltage across Q3 will be roughly the same as Q1's base-emitter voltage, so the output taken from Q2's collector will be very close to the trimpot's input voltage.