# amplifier question

Discussion in 'Electronics Homework Help' started by the_way, Apr 12, 2010.

1. ### the_way

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Mar 12, 2010
hi......I need help with understanding why when i change capacitor C5,C3 to higher value my gain is not stable. i mean when i increase the input frequency my gain will increase but at current or smaller value the gain wont change.

i want to know why , and how to calculate the value mathematically?

2. ### NickS

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Apr 6, 2010
I may be blind. I can see C1, C2, C3, C4, C6, C8 but no C5.

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
The image is too small to be easily readable. I presume the capacitors in question are the ones bypassing the emitter resistors in the circuit.

(much stuff removed because this looks like homework.)

What is the purpose of those capacitors?

Will their effect alter with frequency?

Will DC stability be affected?

What is the DC gain of each stage and how much might it vary (and why)?

4. ### the_way

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Mar 12, 2010
What is the purpose of those capacitors?>>>that is my question???
at very high and low frequencies..
not sure but i think no
first stage around 7 the 2nd is around 5, depends on the biasing ..

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
What is the purpose of this? Is it homework?

6. ### the_way

8
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Mar 12, 2010
design audio amp to drive 4ohm speaker.
is there any way to increase the gain even more.

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
I find it odd that you're using an op-amp to drive the speaker (what is it exactly?) and using a 2 stage transistor amplifier.

The values for the resistors seem odd too.

Where did this design come from? What are the design parameters? (input impedance, gain requires, input signal amplitude, distortion, etc). Is there any particular reason you don't use an op-amp (low noise perhaps) for the pre-amp?

8. ### the_way

8
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Mar 12, 2010
actually i had to design an amplifier using transistors but after i designed the two stages i discovered the impedance matching problem so i tried to use emitter follower to fix that but the transistor used to get very hot and the supply voltage dropped to around 7v. so it was either i add op-amp for the buffer or i change the circuit to class ab and am not sure how to do that.

9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
Just tell us what the assignment question is. You've avoided answering direct questions so often that it just *has* to be homework.

10. ### the_way

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Mar 12, 2010
it is a homework , but i did work on it i just have few problems and i just need help and so advices if there is any .

11. ### the_way

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Mar 12, 2010
i told you the question is to design an amplifier and as u can see i did do it but i would like to improve it

12. ### NickS

367
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Apr 6, 2010
Are you sure there were no other details that you have left out? Your instructor said "build an amplifier" and left it at that?

When we did this assignment in school it came with a large list of parameters relating to Gain, stability frequency response and power consumption. We were also told that it needed to be a two stage MOSFET amplifier(no IC's).

Perhaps you could try to directly re-state the whole question so we can help you without a blindfold and one hand tied behind our back.

13. ### jeanlee411

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May 1, 2010
What is the purpose of those capacitors?>>>that is my question???
at very high and low frequencies..
not sure but i think no
first stage around 7 the 2nd is around 5, depends on the biasing ..

Last edited by a moderator: May 1, 2010
14. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010

You seem to have developed a very odd solution to the problem.

If you had originally designed a class A amplifier capable of delivering 4W into a 4 ohm load, then I'm not surprised it ran hot. So that's not necessarily a fault.

15. ### vk5ajl

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Feb 1, 2010
Gain is not set by biasing. Biasing will change the type of amplifier from class C (no bias) to class B (just biased on) to class A (fully biased so the transistor is on for each full cycle). Class AB is push/pull or, of you like, two opposing class B amplifiers usually with a PNP and an NPN in the output stage but doesn't have to be. This amplifier is class A by the way. Class A amplifiers are the least efficient and so transistors usually do get hot. In this amplifier, the voltage gain is set by the impedance difference between the emitter circuit and the collector circuit. Because the caps across R4 and R10 have lower impedance at higher frequencies, the impedance ratio is higher at higher frequencies therefore the gain. You don't use opamps to power speakers. Tha's just crazy but, in this case, you would do better to just ditch the transistors and the rest of the circuit altogether and just use the opamp.