# Amplifier Design

Discussion in 'General Electronics Chat' started by ayesha91, May 27, 2011.

1. ### ayesha91 Thread Starter New Member

May 23, 2011
6
0
Hello all,,,

first I'd just like to clarify that this is not me trying to let you do my project or anything like that, as a matter of fact I am actually done demonstrating my project yesterday (whew!) However for my project I mostly used a trial-and-error kinda approach but not entirely of course.

For the project we had to design a one-stage BJT or FET amplifier circuit with a dB gain of 50 and a bandwidth 100Hz - 10MHz. and then use this design to build a two-stage amplifier circuit with a gain of 100 and the same bandwidth.

My questions are:
1) How do we usually approach such a design problem (for the single-stage problem)? considering I chose the voltage divider common emitter configuration and BC182 BJT. What calculations are included to reach such a gain and cutoff frequencies?

2) Why will we most likely use a BJT not an FET transistor for such a design?

3) In our design for the two-stage amp we used an op amp buffer or an emitter follower stage between the two stages, Why do we usually do that?

4) And finally, when we used an op amp buffer between the two stages the gain was higher, why is that and is this normal? Also when we used the BJT buffer (emitter follower) between the stages the upper cut-off frequency was lower than the op-amp case, why is that?

THANKS ALL! ^^

2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
To give us a shot at answering your questions, how about posting your circuit design so that we can have a look at it.

hgmjr

3. ### ayesha91 Thread Starter New Member

May 23, 2011
6
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Here you go (sorry!)

50 gain circuit

100 gain two-stage circuit

4. ### Audioguru New Member

Dec 20, 2007
9,411
896
The circuits you show are wrong.

The first circuit with a single transistor has a voltage gain of 180 to 200 times, not just 50 because it has no load.

The second circuit has the buffer with the base bias resistor value the same as its emitter resistor value so it is slammed on hard and doesn't work.

If the buffer circuit in the second circuit is biased properly (but it is slammed on hard like a piece of wire) then the mid-frequency voltage gain will be about 32400 to 40000.

On another thread or on another website you used an old 741 opamp as a buffer. Its max frequency at full output is only 9kHz because it was designed 43 years ago. Most half-decent opamps today go to 100kHz perfectly.

5. ### ayesha91 Thread Starter New Member

May 23, 2011
6
0
Thanks for the reply Audioguru,,, I just wanted to point out that the voltage gain is 50 dB so around 316 times.

Can you please let me know how to properly bias the buffer in the second circuit?

6. ### Audioguru New Member

Dec 20, 2007
9,411
896
I biased all three transistors properly so they have plenty of voltage swing.
You should re-calculate the value of the capacitors because they are too big.

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