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1/25/18

BJT Common Emitter Stage - #1

I've received a few emails over time requesting that I post some simple transistor gain stages that could be used for various audio projects. So, I will be posting a series of transistor circuits starting with this one. The first series of circuits will all operate on +9 VDC.

This single stage BJT (bi-polar junction transistor) stage has been bench tested to verify that the design works. Specifications are given below. Use a trim pot for R1 to bias the circuit.


BJT STAGE SPECIFICATIONS

The gain AV for this circuit, unloaded, is about 16db or 6.3 volts per volt. Is is the current draw in microAmps. Voltages for collector, emitter and base, VC, VB, and VE, are all given. Resistor values are given with notes. Input impedance, Zin, and output impedance, Zout, are also given. Notice that the input impedance, is pretty low at 4.8 kohm. RL = open represents my oscilloscope probe resistance at 1 Mohm.

Specifications for this stage:

AC GAIN

Also notice that the AC gain AV V/V drops quickly once a real load has been placed for "RL". Notice that with a load of 5 kohms you only get about 6db of gain. Load value matters.


SPICE DC VOLTS

The circuit draws about 700 micro amps from the supply. Notice that the collector voltage is set to 1/2 VCC, or 4.5 VDC. SPICE simulation agreed fairly well with the real bench tested circuit.



HIGH BETA TRANSISTOR

The transistor used in this example is an NPN 2N5089 with a Beta greater than 300. Almost any transistor with a Beta greater than 300 should bias up pretty close this this circuit with a little adjustment of the trim pot.



ADJUSTING BIAS

Adjusting bias is easy when you use a quality potentiometer. I use Bourns pots for prototyping.


Thanks for reading,
Trenton

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