# Common Emitter Amplifier design help?

#### Velocity

Joined Feb 27, 2009
14
Hello all,

I'm in an advanced physics lab course which is more or less an electronics lab. The textbook and student manual we use are from Horowitz/Hill's The Art of Electronics. I will freely admit that I am no dab hand at electronics, less so with the design behind them.

Thus my latest assignment has me a bit befuddled. It doesn't help that the professor of the course does not really explain things clearly, nor is the textbook/manual clear and easy-to-understand. I'm not simply looking to get the problem solved; I'd also like to understand the process so I don't have to continue asking for help.

Anyway, on to the problems.

1) Design of an emitter follower with capacitive input and output coupling, using a 2N3904 transistor. Parameters are:

Quiescent current: 1 milli-amp
Signal range: 100Hz-20kHz
R-out of the source to the circuit: 10kΩ
Supply voltage is single-ended, +15V to ground

2) Design of a common emitter amplifier. Parameters are:

Vcc = +15V
Signal range: 100Hz-20kHz
Gain: -50
Quiescent current: 1 milli-amp
Supply voltage is single-ended, +15V to ground

Additionally, a voltage divider allowing me to reduce a function generator output by a factor of ten (the generator is the input for this particular circuit).

Now, I've completed #1 (I think), and I've attached the work for that so that it can be evaluated/checked for errors. #2, however, has me entirely confused at the moment. I know that the resistance of the collector will be 50x the emitter resistor. The 3dB-point for frequency should be <100Hz. I'm trying to follow through the book(s) but it's not really clear to me. I don't have much as far as calculations for #2 yet, so any pointers before I get going would be great. (Again I'm looking to understand the process and the mathematics behind it so I can do it myself without feeling bamboozled.)

Thanks for any help!

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#### Audioguru

Joined Dec 20, 2007
11,248
Your circuit is fine except is missing an output coupling capacitor.
The 23.5pF capacitor should be 1000 times more (23.5nF), check your math.

#### Velocity

Joined Feb 27, 2009
14
Your circuit is fine except is missing an output coupling capacitor.
The 23.5pF capacitor should be 1000 times more (23.5nF), check your math.
Knew I'd forgotten something. This last coupling capacitor, with C = 1/(2*π*f*R) ... I'm assuming f-3dB is the same for it, but the value of R... Would it be ß*Re in parallel with the two large resistors? Wait, that was the input coupling capacitor, wasn't it...

#### Velocity

Joined Feb 27, 2009
14
Yeah, in calculating C1 for the emitter follower, I had the math completely right, I just screwed up on the units. Forgot that it was NANO that was the -9 power and that PICO was -12. My bad.

Regardless, I'm uncertain on how to proceed with C2. The text only talks about it when C2 is linked in series with another resistor. However, the problem I've been assigned does not stipulate having another resistor. Should I include one (arbitrarily or does it depend on something else?), and if not, how do I determine C2's value?

#### thatoneguy

Joined Feb 19, 2009
6,359
That is an excellent book.

Look at page 116 of the Student manual for determining the value of the emitter bypass capacitor.

If your page 116 looks nothing like mine, I can scan it for you, it is X5-2.

#### Velocity

Joined Feb 27, 2009
14
That is an excellent book.

Look at page 116 of the Student manual for determining the value of the emitter bypass capacitor.

If your page 116 looks nothing like mine, I can scan it for you, it is X5-2.
Okay, I see what page you mean here and that will help once I do the common emitter amplifier. However, the circuit you see in the PDF is only the emitter follower from problem #1. The textbook mentions a resistance in series with the output capacitor of an emitter follower. If I want an output capacitor, is it necessary to have the resistor in series with it?

#### thatoneguy

Joined Feb 19, 2009
6,359
For an output capacitor, it would depend on the following stage, which would be C1 in your circuit if it was the second stage. They are looking for a 10k output impedance, which is effectively just Rc.

For the second problem, without looking at the teachers manual and cheating, I don't see enough information (input level/impedance, output impedance)

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