Sunday, November 17, 2019

Another look at the Transistor Universal (Amp) P or N

We start with a general purpose transistor and 4 resistors. The resistors I'm using are 100k, 10k, 4k7 and 511. I use these because they are what I have on hand you could use what you have within a reasonable amount. As we look at the circuits you will see how critical the values are.

First we establish the DC bias. R3 and R4 set the base bias. R2 sets the emitter bias. R1 is the collector load resistor. Connecting the Vcc completes the DC circuit.

Adding the capacitors provides the AC circuit. I marked the possible circuit connections. The The base cannot be the output and the collector cannot be the input. We connect the signal to the input and the load to the output. The third capacitor goes to ground (-Vcc).
Completing the circuit we have a battery and signal source. Time for some sim's.
Input base, output collector and ground emitter.
Input base, output emitter and ground collector.
Ground base, input emitter and output collector.

You may see the circuits drawn differently. Compare these three. In all three the base is grounded and the input is on the emitter. I downloaded these next three to show other variation.

So all we need is 4 values of resistors and a general purpose transistor to establish a basic amp stage with DC bias. Three capacitors to set the AC circuit response. The 2N3904 is a good general purpose transistor for hobby work. It will work in circuits up to 100 Mhz. You can look at the post on TUN and TUP circuit to see the tables of transistor you can use.


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