Beginner Amplifier

Here is the explanation behind a basic, single transistor, voltage amplifier. Great starting point for beginners. This is a "how to" on creating a basic class A amplifier. The configuration shown here may differ from your expectation. Unfortunately I cannot cover all of the different configurations at once. Working from the top left to bottom left. Just like reading this sentence. Here is the break down of each circuit. 1. The goal here is to set the BJT collector voltage to 1/2 of the input voltage. Vin/2 ~ Vin-(100 ohm)*B*(Vin-0.8)/(50k ohm) This allows the transistor to operate in the forward active mode. 2. We examine the maximum input current. Vin/2 - 25uA*B*(100 ohm) = 1.75v 3. We examine the minimum input current. Vin/2 + 25uA*B*(100 ohm) = 3.25v 4. Replace the current source with a AC source. Use a cap to block supply current form entering the AC source and for other reasons. Use a resistor (10k) to set a limit on the current. Imax = Vs/10k, Imin = -Vs/10k 5. Replace the resistor (10k) that was used for limiting the current with a potentiometer. Now, you can adjust the input current and therefore adjust the voltage gain. 6. An additional high pass filter can be placed on the end of the amplifier for later use or to easily see the voltage gain. I hope this has helped you better understand class A amplifiers. Questions to think about. 1) What if you take 2 amplifiers and connect them in a loop so that the input of an amplifier is connected to the ouput of the other? And what about 3 in a loop? 2) What is the overall gain? Is there an upper limit? 3) What frequency range can we amplify? 4) What happens if you shift the output another 180° using capacitors or inductors and connect that output to the amplifier's input? 5) What does connecting a capacitor from the base of the BJT to ground do?

published 9 years ago