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Design >> Analog Design >> audio amplifier design
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Message started by aman gupta on Oct 14^th, 2012, 7:17am

Title: audio amplifier design
Post by aman gupta on Oct 14^th, 2012, 7:17am

I am studying basics of analog circuit design from microelectronics circuit analysis by donald neamen
And in that i encountered a problem in design of audio amplifier.
The question and solution is given in the attachment.
And i want to know that in the output emitter follower why there are no biasing resistors apart from emitter resistor ?
Hope my question is clear and if not then please do tell me. I want to clarify this
Thank you

Title: Re: audio amplifier design
Post by buddypoor on Oct 14^th, 2012, 8:06am

And i want to know that in the output emitter follower why there are no biasing resistors apart from emitter resistor ?

My interpretation of the diagram is as follows:

An input dc bias current is shown (Ib arrow) which means that, of course, a base-emitter dc voltage of approx. 0.6...0.7 volts is necessary.
However, a corresponding resistive divider is not shown - but this does not mean that it shouldn`t exist!
As another solution the corresponding bias is provided by the dc output voltage of the preceeding gain stage, which in this case must be dc coupled to the output stage.

Title: Re: audio amplifier design
Post by ywguo on Oct 15^th, 2012, 6:14am

Hi aman gupta,

Why does it need additional biasing resistor apart from the emitter resistor? The emitter resistor and the base voltage determines the bias current, I_E.

BTW, please make your question more clear. I guess resistor R_E1 Figure 6.77 is what you said emitter resistor. Am I correct?

Best Regards,
Yawei

Title: Re: audio amplifier design
Post by aman gupta on Oct 15^th, 2012, 7:48am

ywguo wrote on Oct 15^th, 2012, 6:14am:

Hi aman gupta,

Why does it need additional biasing resistor apart from the emitter resistor? The emitter resistor and the base voltage determines the bias current, I_E.

Best Regards,
Yawei

I am refering to emitter resistor RE4
in fig 6.78........... I was asking why we can't use voltage divider biasing in emitter follower in fig 6.78

But i think what buddypoor said, makes sense.
So is the operating point of emitter follower in fig 6.78 is being set by gain stage ???

Title: Re: audio amplifier design
Post by raja.cedt on Oct 15^th, 2012, 8:07am

hello,
please refer to a basic electronics book. or else watch vedio's from thsi website.

here is the explanation: Yes you can use potential divider, but you have to AC couple the bias coming from the previous stage. But why you want a resistive divider when you are having bias from some where....

The idea of resistive divider is to get the rite voltage at the gate of the transistor, when you have higher than required you are well come to do all those things, but fortunate you have bias from previous stage...

Thanks,
raj.

Title: Re: audio amplifier design
Post by aman gupta on Oct 15^th, 2012, 8:17am

i got it...
thanks everyone
this site is really helpful.........