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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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Posted by Ronelex 6 Responses

6 Responses to “Getting Bias Current of an Output Transistor of an Amplifier Circuit”

  1. masood says:

    dear sir
    today i came to know tha this type your you suggest bias voltage 20mv for 60w amplifier,how can you find 20mv or 50mw .why not 60mv or 100mw.your forum is good and interesting thanking you.

  2. DjMarBMC says:

    Ron do you have an article regarding on how to check or to fix dc out voltage in a power amplifier circuit?

  3. xzdragon says:

    pano kng mag check ng dc off set nya?\

    • andhoie says:

      set mo sa 2.5 dc volts ung tester at bago mo tusukin ung speaker terminal make sure n naalis mo n ung input connection I hang mo ung input depende kung nilagyan mo ng volume control I cut mo lang ung positive line then kelngan ang reading pag pinihit mo trimmer .5 or lower…much better I trial and error mo nlng replace it with resistor…I cut mo ung linya nia sa pcb jumper mo ng resistor sa akin kc 18k ohms ang nag match

  4. andhoie says:

    actually na perfect ko ang thunder 100watts ..I use it on my mobile more than 7 yrs without crushing…….ang gnaw ko ung trimmer nia pra sa dc output…nag trial and error ako gnaw kong resistor …I use 18k ohms…depende in sa ampli s akin 18k xa umige…


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