I'm new to the forum and circuits/ DIY amps in general, but I've been doing a Music Tech course and have been set some questions to resolve. I need to increase the voltage gain of a guitar practice amp so I can use a guitar pickup with quite a small input voltage.

The two resistors in the pre-amp stage are 10k and 470k and if I reduce the 10k to 1k and increase the 470k to 1M, I adversely affect the frequency response and the treble tone control stops working properly.

I realise this is because the break frequency has changed, but am not 100% sure of the details or how I can find a better way of increasing the voltage gain. I recall from my notes that I could chain the amp stages together ( is it 'cascading' or 'bootstrapping'?) or maybe increase the power supply.

Any help would be great...


Cheers!

 

Can you post the schematic of just the section under discussion here in the forum?

hgmjr

 

Thanks. Unfortunately not...I only have it on paper and don't have access to a scanner. But I only need to describe in fairly general terms what happens when the ratio between the resistors has changed in the way described, and suggest better ways of increasing the voltage gain.

 

For a simulation program, there is a free one at http://focus.ti.com/docs/toolsw/folders/print/tina-ti.html

You can draw your schematic in that program and then print it to a pdf file. There is a free pdf distiller at http://www.cutepdf.com

I'm sure others will recommend other schematic capture programs.

 

To answer you quickly, yes. If you increase your gain you will lose bandwidth especially in voltage feedback amplifiers. It is impossible to decouple the signal gain from an amplifier's frequency response. This is where the concept of the gain-bandwidth product comes from.
Current feedback amplifiers are ideally independent from this phenomena; i.e., their stability is soley determined by the feedback resistor and the bandwidth is not dependent on signal gain. You can increase gain yet still maintain a large bandwidth.