COMMON EMITTER amplifier urgent help needed

Thread Starter


Joined Dec 3, 2009
A common emitter Class A audio amplifier is to be designed that must meet the following specifications:
1. Power supply (V
CC) as 12V.
2. Voltage gain at 120 when V
i is a 1 KHz sine wave with amplitude of 50 mV peak to
3. BC108 data for h
FE has values between 110 and 800
4. Bandwidth set at 100 Hz to 20 KHz.

DEADLINE TOMORROW, ANY HELP WOULD BE GREATLY APPRECIATED, am required to calculate component values and state any assumptions made.



Joined Nov 21, 2009
First, you need to bias the transistor properly so that you don't get clipping. The gain is 120, so that means for a 50mV p-p input you need a 6V p-p output. So, the transistor bias point needs to be offset so it is above +3V DC and below +9V DC, otherwise clipping occurs. So biasing the base somewhere near 6VDC might be a good choice. The Ci and Co coupling caps are there to block the DC bias voltage from the input and output of the amp. Choose large value coupling caps so they don't block 100Hz input/output. You need to keep the transistor operating inthe linear region, not in saturation or cutoff, so choice of RL is used to set the collector current, and thus partly selects the operating point. The Beta (hfe) varies depending on the collector current magnitude. You want the base current multiplied by hfe to be no more than the maximum collector current as set by RL. Circuit analysis will give you the input impedance once you choose some values for the base bias resistors and the emitter resistance. I think I've given a few hints, good luck with your studies.
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