Basic Darlington Amplifier

Connecting two transistors as a Darlington pair by connecting the emitter of the first transistor directly to the base of the second transistor multiplies the beta of the first transistor by the beta of the second transistor to give an extremely high-gain device. Imagine, for instance; two NPN devices, each with a maximum gain of 50, connected in such a way, giving a device with a maximum gain of 2500: This would be useful for boosting the output of a piezo-microphone for example; notorious for its low output. In the basic circuit-diagram of a Darlington amplifier below; the input is DC decoupled by C1, resistors R1 and 2 from a potential divider biasing the base of Q1 at exactly 0.7 Volts, R3 is the load resistor on the collector of Q1 – which drives the base of Q2; R4 restricting its collector load, and the R4/C2 combination decoupling its emitter to ground.

Since the base of Q1 is at 0.7 Volts, both Q1 and 2 will be in an “always on” state, and sensitive to any tiny ripple passing through the input capacitor C1. Bearing in mind that the input ripple will probably be of only a few microamperes; the R1,2 pair should be selected with as high a resistance as possible – Within the megohm range to limit the current already present on the base of Q1 to a fraction of a microampere if at all possible. Q1 should be chosen such that its base current need only be negligible for it to respond. With a beta of 50 the resistance of R3 should be within the range of around -2 to -40 times that of R1, so as not to drive the transistor into saturation. Again, having a beta of 50; Q2 should be run ideally at between 2 and 40 gain. Suggested component values to run the circuit at a voltage of 1.5 Volts are as follows:- R1: 2M2, R2:(1M with 250K Lin. Preset in series.) R3: 100K, R4: 22K, R5: 110R C1: 1uF 10V Elect., C2: 100uF 10V Elect., C3: 10uF 10V Elect. Q1: BC107B, Q2: BC109C Note: I haven’t built this circuit myself; and it’s been drawn up for demonstration purposes only: It’s very basic and wouldn’t give brilliant sound quality anyway, but should nevertheless “work”.