# BJT Common-Emmiter Amplifier - headroom question

#### DynastyFury

Joined Nov 4, 2011
1
Hello, I'm new here. Please let me know whatever or not I can post here.

I'm still learning circuits and need help. I'm stuck on configuring the 2nd stage of my multistage BJT amplifier. The problem is understanding the headrooms for the circuit. After this, I can do the first stage.

Specifications of mutistage Amplifier:
30 Gain
600 mVpk output
Linear so that harmonic distortion is under 25 dB
input inpedance > 50 kΩ
Supply voltage 5 V
Must not be greater than 3 stages

Using these specifications, I decided to use 3 stages: a CE, CE, and CC. The 3rd stage (common-collector) is a buffer with gain of near 1. The 1st stage has a gain of 6, and the second stage has a gain of 5.

Working backwards, the 3rd stage is done. Now, I am on my second stage. This is where the problem occurs. The input inpedance going into stage 3 and the load of stage 2 is 675.26 Ω. The output voltage swing of stage 2 is 610 mVpk.

Attached contains what I got so far for calculations.

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#### t_n_k

Joined Mar 6, 2009
5,455
The stage design has some issues. Particularly with the biasing arrangements. Also it would be informative to take a look at what you've done on the other stages rather than giving a piecemeal (stage-by-stage) summary of the design.

What have you done on the output 8Ω load driver? Have you used a complementary pair or is that not allowed?

#### hobbyist

Joined Aug 10, 2008
889
Just because your design can be no greater than 3 stages, don't limit yourself to only 3 transistors.

A stage does not have to have only one transistor, some stages are designed with several transistors, you can combine a CC with a CE with direct wiring (no capacitor coupling), and have that as a single stage.

Because the CE configuration will be used as the Voltage bias for the base of the CC configuration, replacing any bleeder resitor that could be designed into the CC base bias network.

So by combining the CE and CC direct coupling you would have a low output impedance with a small voltage gain, and because it is direct coupled would act as one stage, namely the output stage.

You could use two transistors coupled as darlignton pair for your CC and it would be consisedered one stage,

so a three stage amplifier is a amplifier that has 3 stages of signal manipulation, not just only three trtansitor configurations.

A good way to determine the stage perimeters, is where the signal sees a coupling, via, capacitance, inductance, and even direct coupling under certain design considerations.

And where the signal is manipulated (amplified, impeded,changed in frequency,(hetrodyning),
ect.

So be creative in your design.