pre amplifiers

[the cracken]

why is it that an opamp with a + - 15 supply and a gain of 5 and an input voltage of 3 will not give you 15v peak to peak???
is it something to do with the input resistance??
iv seen that on a lot of circuits they have a transistor before the opamp why is it there?
thanks for any help

 

[Nigel Goodwin]

Why would you think an opamp can't do that?, it's a pretty basic application and it will do it fine.

I've no idea what you mean about a transistor 'before the opamp' either?, perhaps you could post an example?.

 

[#12]

Most op-amps can not provide output at the absolute highest and lowest voltage available. Some of the newest op-amps have "rail to rail" output. You might buy one of those if you want +/- 15.00 volts output.

Why a transistor before an op-amp is too vague. Many purposes.

 

[MRCecil]

Is your 3V input 3VP-P, 3VP or 3Vrms?

If it is 3VP-P, then it should be just fine as Nigel said. If it is 3VP, you will have to either go to a rail-rail op amp as #12 said, but raise the supply rails by a few hundred mV to avoid all clipping of the signal OR raise the supply rails to overcome the limitations of your existing op amp. If it is 3Vrms, then your looking at an input of nearly 8.5VP-P so a gain of 5 would yield a potential of ~42.2VP-P, which puts the effective value (rms) of the output right at the rail.

 

[the cracken]

i can't actually find the circuit i was looking at. so i have an electret mic which is about 1.2k or something how do i know the capacitor and resistor size i need for it and what kind of voltage can these thing run on??? i made the circuit with a 1k resistor and a 1mic cap with a 741 opamp and a gain of about 6. i got a big 50Hz sign wave and a very weak signal.

 

[the cracken]

yer the new fet ones. it wasn't anywhere near what it should have been (i was using a sig gen for the input) so could you give me some examples of why you would put a transistor before an opamp?? would one reason be impedance matching??

 

[RMMM]

Do you mean a transformer before the pre-amp?

I am struggling to understand. It would be helpful if we could see what you had in mind.

 

[Nigel Goodwin]

yer the new fet ones. it wasn't anywhere near what it should have been (i was using a sig gen for the input) so could you give me some examples of why you would put a transistor before an opamp?? would one reason be impedance matching??

Like I asked before - you need to give US an example, it's not something that's normally done, and it's VERY unusual.

The only 'common' (still fairly rare) application is to use two transistors at the front for a low-noise balanced microphone preamp - some professsional mixers do that, but most don't. Presumably it's to get a lower noise factor while using a cheaper opamp?.

As for your electret mike preamp - there's an example in the 'sticky' at the top of this very forum (under basic opamp circuits). Your loud 'buzz' was because you build it wrongly, so it was picking up mains hum.