Microphone Amplifier

[Nigel Goodwin]

i thought a class A amplifier used a transistor that conducted for 360 degrees and an output capactor to get the positive and negative swing. as opposed to the push pull arangement of the class B ?

No, it's merely a question of biasing.

BTW, a point Audioguru didn't mention, the most important function of the emitter resistors is to help prevent thermal runaway.

 

[Gaston]

so you can have a class A amp with a push pull arangement?

 

[audioguru]

BTW, a point Audioguru didn't mention, the most important function of the emitter resistors is to help prevent thermal runaway.

I think there will not be any thermal runaway if the diodes are mounted on the heatsink so they are at the same temperature as the output transistors. A transistor with a heatsink tab performing as an amplified diode follows the temperature of the output transistors better than diodes.

 

[Nigel Goodwin]

I think there will not be any thermal runaway if the diodes are mounted on the heatsink so they are at the same temperature as the output transistors. A transistor with a heatsink tab performing as an amplified diode follows the temperature of the output transistors better than diodes.

There will be without any emitter resistors, they are of more value than mounting the diodes (or transistor) on the same heatsink.

 

[joeypc]

The circuit is designed for 4W output so it uses a high supply voltage and many transistors for high current. A 0.4W amplifier would use a lower supply voltage and only 3 transistors.

0.4W RMS is a peak-to-peak voltage of 5.1V and a peak current of 318mA into 8 ohms.
If the amplifier has a power supply voltage of about 7.5 V and the output transistors are 2N4401 and 2N4403 then the max RMS output power into an 8 ohm speaker will be around 0.4W.

I simulated it. It has only 3 transistors so its distortion at 0.4W is about 10% and is lower at lower levels. Its voltage gain is about 18.

Have you tried out this circuit yet ? Could I replace 2n4401=2n3904 and 2n4403=2n3906 ? Well could you have any modification so that the circuit will be able to work with 9 volts battery ?
Thanks !

 

[audioguru]

Have you tried out this circuit yet?

It is an extremely simple circuit that has neen made by thousands of people. But it has a high amount of negative feedback to keep its distortion low. Then its voltage gain is low.

Could I replace 2n4401=2n3904 and 2n4403=2n3906 ?

Make a quick calculation of the max current.
With a 9V battery the max current in the output transistors is about 406mA.
Then look at the max allowed current for a 2N4401 and 2N4403, 600mA so they are fine.
Then look at the max allowed current for a 2N3904 and 2N3906, only 200mA so they will blow up.
Guess why i selected the 2N4401 and 2N4403?

Well could you have any modification so that the circuit will be able to work with 9 volts battery ?

It will work fine from a new little 9V battery for a few minutes. But the voltage quickly drops as the tiny battery runs down. When the battery is new then the power output is 660mW.

 

[fsmyth]

If you are still working on this,
there is a collection of mic drivers at:
https://hmin.tripod.com/als/fsmyth/pages/audio-2.html
<als>

 

[Hero999]

Don't forget that there's a difference between peak current and continuous current, most transistors can handle double their constant current rating for short periods of time.

The chances are a transistor rated for 200mA won't be damaged by a short 406mA spike every now and then. Also, the current won't be anywhere near 406mA as the battery voltage will drop considerably when the load tries to draw this current.

 

[audioguru]

Don't forget that there's a difference between peak current and continuous current, most transistors can handle double their constant current rating for short periods of time.

The chances are a transistor rated for 200mA won't be damaged by a short 406mA spike every now and then. Also, the current won't be anywhere near 406mA as the battery voltage will drop considerably when the load tries to draw this current.

One function of a supply bypass capacitor is to supply high current to the circuit when it is needed.
If the amplifier is playing extremely distorted music (square-waves) like Acid Rock then the poor little output transistors will be operating at double their max allowed current for half the time.

 

[Hero999]

If the amplifier is playing extremely distorted music (square-waves) like Acid Rock then the poor little output transistors will be operating at double their max allowed current for half the time.

Exactly, for half the time, therefore the average power dissipation will be equivalent to half the applied current.

 

[joeypc]

Well... Thanks audioguru for your answer(s). Uhm... I have another questions. Why do we have to add the 5.6K ohms resistor at the input ? Another question is about the Push-Pull transistor biasing. Well why do we have to add 27 Ohms resistor ? Well 2 1n4148 diodes will apply a voltage about 0.6v in each BE transistor junction and will that be enough to limit the cross-over distortion ? And is that the reason why you put 27 Ohms resistor to limit the cross-over distortion ?
Thanks !

 

[audioguru]

Why do we have to add the 5.6K ohms resistor at the input ?

Without the 5.6k input resistor, the input impedance is very low which loads down the input source, it causes the input capacitor to cutoff bass frequencies unless the capacitor has a much higher value, and it increases the gain and distortion.

Another question is about the Push-Pull transistor biasing. Well why do we have to add 27 Ohms resistor ? Well 2 1n4148 diodes will apply a voltage about 0.6v in each BE transistor junction and will that be enough to limit the cross-over distortion ? And is that the reason why you put 27 Ohms resistor to limit the cross-over distortion ?
!

The voltage across the two diodes cancels the voltage of the two base-emitter diodes of the output transistors. The 27 ohm resistor increases the voltage enough to also cancel the voltage in the 1 ohm resistors so the idle current is about 10mA to almost eliminate crossover distortion.

Here is the circuit without the 27 ohm resistor:

 

[joeypc]

Thanks audioguru ! Well are there any modifications so that the push-pull transistors 2n4401 and 2n4403 can be replaced with 2n3904 and 2n3906 ?
Thanks !

 

[audioguru]

2N3904 and 2N3906 transistors can be used if the supply voltage is reduced to 6V so the max current is reduced and the power output is reduced to about 0.22W which is almost nothing.

You can buy one 2N4401 transistor today at Digikey for only $.12US and a 2N4403 for only $.13US.
An LM386 amplifier IC costs only $.94US and has much lower distortion.