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BJT Common Emitter Circuit #2

BJT Gain Stage #2

Here is the second common emitter gain stage for this series. It operates on a 9V battery or 9V DC supply and uses a 2N5089 NPN transistor. This circuit has a fairly low input impedance so you won't be plugging your electric guitar directly into this stage unless it has active pickups with low output impedance.


This circuit has been bench tested to verify that the design works. Use a good trim pot (see "Adjusting Bias) for R1 to bias the circuit. SPICE simulation calculated R1 to be about 34k Ohms.

The gain AV for this circuit, unloaded, is about 19.4db or 9.34 volts per volt. Is is the current draw in microAmps at 513 uA. Voltages for collector, emitter and base, VCVB, and VE, are all given. Resistor values are given with notes.

Zin is pretty low at 4.7 kOhm. Zout is about 15 kOhm. RL = open represents my oscilloscope probe resistance at 1 Mohm. (Not truly "Open")

Specifications for this stage:


AC gain for a common emitter stage can be calculated approximately by:

Av ≈ RC total / RE total

EX-1: Av ≈ 15 kohm / 1.5 kohm = 15,000 / 1,500 = 10 V/V

Notice that we neglected the load resistance of 1 Mohm. But, you can't neglect the resistance value for lower load resistances. Let's calculate the gain with Rl = 5kohm.

EX-2  Av ≈ (RC total / RE total)

RC total = 15 kohm || 5 kohm = (15 kohm x 5 kohm) / (15kohm+5kohm)

RC total = 3.75 kohm

Av ≈ (RC total / RE total) = 3.75 kohm / 1.5 kohm = 2.5V/V

Not bad considering the actual measured gain was 2.32 V/V. Remember, take all resistances connected to the collector( Rc in parallel with your load) and divide that by all of the resistances connected to the emitter (In this case just the emitter resistor RE). Gain vs. load resistance is given below.


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 R1.

Note: Transistor "E-B-C" (Emitter, Base, Collector) pinouts for transistors can vary. All 2N5089's will have the pinout given below. But, other transistors might be B-C-E, or C-B-E. Always verify device pinout before building.


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

Thanks for reading,

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