# I need help to design a three stage amplifier using only 2N3906 or 2N3904

Discussion in 'The Projects Forum' started by mercure, Feb 19, 2008.

1. ### mercure Thread Starter New Member

Feb 19, 2008
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I need to design a three stage amplifier to allow this microphone(peak open circuit voltage of 1mV with an output resistance of 10kOhm) to drive an 8ohm speaker in such a way that the output voltage amplitude is maximised. in addition, the following constraints are placed on the design.

1) Circuit is to be biased from a +/- 12v DC power supply.
2) Have a bandwidth of 20Hz - 20kHz.
3) should produce no obervable clipping or distortion in the output voltage waveform when the input voltage is at its maximum value. (1mVpk)
4) Only use NPN 2N3904 or PNP 2N3906 transistors or both.
-2N3904 : B=300, C1=4.5pF, C2 =3.5pF
-2N3906 : B=400, C1=6.3pF, C2 =5.8pF

Anyone can help me with the design of the circuit?

2. ### Audioguru New Member

Dec 20, 2007
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896
1) The load of the amplifier is 8 ohms.
2) A pair of complimentary transistors can produce a signal level of about 10.5V peak into an 8 ohm speaker with a plus and minus 12v supply.
3) 10.5V into 8 ohms is a peak current of 1.3A.
4) The max allowed current of a 2N3904 and 2N3906 transistor is only 200mA. They will blow up with 1.3A. You need many paralleled transistors at the output.
5) The output transistors amplify current, not voltage. Then two more stages must amplify the voltage from 1mv to 10.5V. The total gain must be 10,500 and each of the two stages must have a gain of 102.5 times. They will be extremely distorted if they are single transistors at such a high gain. If you use two transistors with negative feedback and bootstrapping for each stage then the distortion can be low.

It is an impractical amplifier circuit.

3. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
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It depends on how you define the stages, think of them as each being self-contained: you could make a (relatively) low noise mic pre-amp with a gain of 22, and then a line-amp with a gain of 22, and then a driver/output amp with a gain of 22. Gain distribution is relative, of course.

4. ### spar59 Active Member

Aug 4, 2007
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To match an 8 ohm speaker without using parallelled output devices (since you are not allowed to use higher rated ones) you could use a transformer - these were often used in cheap transistor radios with a push-pull output stage though you could also use a single-ended output stage. I realise that this is a bit of a cheat rather than simply using a higher impedance speaker but it would meet the end goal.

Steve.

5. ### Audioguru New Member

Dec 20, 2007
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896
A transformer on the output of a transistor amplifier?
Welcome to 1964.

Edit: Philips made one in 1955. Its schematic is too fuzzy so it exceeds the max limit of this website.

Jan 27, 2009
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hi mercure,
i also searching for making the same amp which you want to do.i have a doc file in which it is described that steps by which v can make that amp.send me your email address so that i will mail it to u.my address is muhammed_adil86@yahoo . v will help each other to make this.thanks