# Class of Tr. Amp.

Discussion in 'General Electronics Chat' started by mkbutan, Nov 8, 2009.

1. ### mkbutan Thread Starter Senior Member

Sep 30, 2008
270
16
what are the classes of Amplifier's (as CLASS A;B;C;D or x; etc.) how to classify the class of any Transistor Amplifier
(eg.:-to take any BJT transistor out there and bias it in order to make it a Class-A amplifier.) as in the thread ( http://forum.allaboutcircuits.com/showthread.php?t=29729 ) by Mr.Austin

2. ### hobbyist Distinguished Member

Aug 10, 2008
764
56
If your extrememly new to transistor amp design, then here is a very BASIC procedure.

Without getting into input and output impedances, but just to bias a transistor into its linear region, using a NPN transistor.

1. choose a resistor value for RC. (the resistor from collector to positive supply VCC.
2. Take half the supply voltage VCC. and divide this resistor into it, to give collector current. (IC)
3. Divide the RC value by 10 and use a resistor close to that value for the emitter resistor. RE.
4. multiply IC x RE to give a value of VE. (voltage at the emitter)
5. Add (VE + 0.7v.) to give voltage at the base (VB). (assuming a silicon transistor)
6. Put a resistor from ground to base that has a value of around 5- 20 times RE. (R1)
7. Take (VB / R1) to give divider current. (ID)
8. then {(VCC - VB) / ID} = the value for the top resistor from base to positive supply, use a resistor close to that value.

The most important part of this design is this,
prototype it (breadboard, or simulator), and check the voltage at the collector with respect to ground, you should have close to 1/2 VCC.

If it is too low, then the transistor is on too hard, so either decrease the bottom resistor, or raise the top resistor value. (of the divider)

If it is too high, then the transistor is not on hard enough, so either increase the bottom resistor, or decrease the top resistor value of the divider.

Adjust these values until you have close to half the supply voltage.

That is called quiescence state, which simply is linear mode (class A) of operation.

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