Microphone Amplifier - Page 2 - Electronic Circuits Projects Diagrams Free

audioguru · 6th May 2008, 11:43 PM

The amplifiers I simulated are class-AB because the output transistors have about 15ma of idle current.

The 1 ohm resistors even out the huge spead between the base-emitter voltage of different transistors.

Without the 1 ohm resistors then the output transistors will either operate in class-B with high crossover distortion or in class-A with a high idle current and lots of heat.

nicho12 · 7th May 2008, 04:43 PM

awesome! you guys are geniuses!
this helps loads..but do these updated circuits still need a preamp? will the previous pre amp posted work?
cheers
N

nicho12 · 7th May 2008, 04:59 PM

oh and btw that stuff about the 3.2ohm speakers coming from an old text book...it actually came directly from my lecturer, who pretty much is an old text book! haha

audioguru · 7th May 2008, 04:59 PM

The amplifier circuit I simulated with the 8 ohm speaker has a voltage gain of 18. Its output voltage at 400mW is 1.79V so its max input voltage is 1.79V/18= 99mV.
A microphone has an output level of from 2mV to 10mV so it needs a peamp with a voltage gain of at least 49.5.

Gaston · 7th May 2008, 05:59 PM

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 ?

**Nigel Goodwin** · 7th May 2008, 06:04 PM

Quote:

Originally Posted by Gaston

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 · 7th May 2008, 06:13 PM

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

audioguru · 7th May 2008, 07:32 PM

Quote:

Originally Posted by Nigel Goodwin

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** · 8th May 2008, 08:41 AM

Quote:

Originally Posted by audioguru

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.

6th May 2008, 11:43 PM	(permalink)
audioguru Experienced Member	The amplifiers I simulated are class-AB because the output transistors have about 15ma of idle current. The 1 ohm resistors even out the huge spead between the base-emitter voltage of different transistors. Without the 1 ohm resistors then the output transistors will either operate in class-B with high crossover distortion or in class-A with a high idle current and lots of heat. __________________ Uncle $crooge

7th May 2008, 04:43 PM	(permalink)
nicho12 New Member	awesome! you guys are geniuses! this helps loads..but do these updated circuits still need a preamp? will the previous pre amp posted work? cheers N Last edited by nicho12; 7th May 2008 at 04:49 PM.

7th May 2008, 04:59 PM	(permalink)
audioguru Experienced Member	The amplifier circuit I simulated with the 8 ohm speaker has a voltage gain of 18. Its output voltage at 400mW is 1.79V so its max input voltage is 1.79V/18= 99mV. A microphone has an output level of from 2mV to 10mV so it needs a peamp with a voltage gain of at least 49.5. __________________ Uncle $crooge

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7th May 2008, 05:59 PM	(permalink)
Gaston Experienced Member	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 ?

7th May 2008, 06:13 PM	(permalink)
Gaston Experienced Member	so you can have a class A amp with a push pull arangement?