Type of transistor configuration

Thread Starter


Joined Mar 24, 2016
What type of configuration are transistors Q1 and Q2 in? What is the advantage of this setup, i.e., why not just use a single NPN transistor?
Relay driver.jpg


Joined Aug 1, 2013
Q1 and Q2 both are operating in a special case of a common-emitter circuit called a saturated switch. Each one acts as an on/off switch based on the voltage at and current through its base. The most current each collector can pass is determined by the gain (Hfe) of the transistor. The designer of the circuit determined that one transistor's gain was not enough to reliably boost the current from the optocoupler high enough to drive the relay coil.

In this circuit, Q1 gets about 0.5 mA from the optocoupler and boosts it to about 11 mA through the Q2 base. Q2 amplifies this to drive the relay coil and LED. My guess is that those two loads combine for less than 200 mA.

You probably could do everything with one transistor, but it would mean that the circuit input needs much more current from whatever source is driving it, like maybe 10-20 mA instead of approx. 1-2 mA. The 4N28 optocoupler is not a very efficient device, with a current transfer ration (CTR) of only 30% typical. This means that for every mA into the LED side, only about 0.3 mA flows through the output transistor.

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