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Getting Bias Current of an Output Transistor of an Amplifier Circuit


Ronelex Electronics/Biasing Current of an Output Transistor

In this article, we will be focused on getting the bias current of an output transistor of an amplifier circuit. According to wikipedia:

Biasing in electronics is the method of establishing predetermined voltages or currents at various points of an electronic circuit to set an appropriate operating point. The operating point of a device, also known as bias point, quiescent point, or Q-point, is the steady-state operating condition of an active device (a transistor or vacuum tube) with no input signal applied.

This is the method shared by ricdelros, one of the Elab Forum member, based on the topic about how to get the bias current of an output transistor” Bias voltage can be simply check by following situations that will serve as a guide.


1. Power-up the amplifier circuit without Output Transistor.

Having a driver transistors can provide a few hundred milliwatts of power without the Output Transistor which should be enough to tell if an amplifier operation is correct. If it’s going right so far, turn of power and connect the output transistors, turn the bias trimmer to max resistance, connect 8 Ohms load and apply power again.

Example: If you have 60w amp, connect a DC Voltmeter across either of Output Emitter Resistor and adjust Bias Trimmer and adjust trimmer for a reading of slightly less than 20mv, this will set an Idle Current of around “50mA” which is close to specs of 60w. Watch your multimeter while testing. If at anytime the reading jumps when you have no input, “”STOP”” and check quickly for circuit oscillation, if you are not lucky, such condition may blow fuse, amplifier with this kind of problem can cause heartburn and lots of ulcer and stress. Recheck the circuits..

Bias Current


2. Another way is to check Both Emitter Resistor voltage of the output transistor. Connect a Positive lead to Point M and a negative lead to Point N and advance the idle by trimmer until you get a voltage reading of 20 millivolts below.



3. Other very simplest way is to connect a Milliammeter between the Power Supply Source and to either Positive or Negative Rail Terminal (must have a firm connection or else a little loose can blow the opposite fuse). You can see the the idle current here of 50mA as registered in step 1. This method is the one we used in testing the idle current taught by our instructor while I’m studying electronics.

Bias current





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