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low output impedance signal amplifier

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IamSmooth

Tue Oct 06 2009, 10:24PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Are there any models out there that can be purchased, or is there anyway to convert a standard signal amplifier to one with a low output impedance?

Is it as simple as putting the signal through a buffer?

Myke

Tue Oct 06 2009, 10:52PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Op-amps have a high input impedance and a low output impedance. You can connect the output to the inverting input for a simple follower or you can put resistors on the input and feedback to make it an amplifier.

What's your application?

EDIT: I didn't know it was referencing your earlier thread. You would need an RF amp which isn't the easiest to build but can be done. Op-amps don't do so well with RF freqs because the maximum gain of the op-amp decreases as the freq increases. So no, an op-amp won't work for your application. You can put a resistor in series with a parallel LC tank to increase the overall impedance. Try a piece of nichrome wire for the resistor in the tank for sensing. I'll have a high enough resistance to show up on your scope and a small enough resistance not to make the Q factor fall too far. I'm not sure how much you would need though.

IamSmooth

Tue Oct 06 2009, 10:58PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

The application goes back to the thread were I want to make measurements on LC circuits that have a low characteristic impedance. My current signal generator only goes to 200kHz. I plan to get one that goes to 20 mHz.

In the meantime, my amplifier roughly has an output impedance of 35-50R, which affects my measurements. What seems strange is if it is so simple to add an op-amp, why isn't one built into the unit? Will I distort my signal, or will it not respond to the high frequencies?

Dr. Slack

Wed Oct 07 2009, 07:45AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The output impedance of a signal generator will affect your measurements, but in a calculable way. It is often more useful to have an impedance which approximates 50ohm, than one which approximates 0. This is increasingly important as the frequency increases, with 200k you can probably get away with low impedance, with 20MHz it's much better to have a 50ohm output. In the first instance, try to rethink your experiement so that it can tolerate a 50ohm drive impedance.

If you really want a very low impedance for some measurement, then it's probably better to convert right at your experiment with a low impedance buffer, or a small resistor to ground, or a step-down transformer, and have a 50ohm cable going back to your 50ohm signal generator (first hint as to why that output impedance!).

There are some fvery fast buffer amps on the market if you really want to build a general purpose high speed buffer. Maxim IC, who have a policy of providing 2 samples of each part number to whoever asks, especially amateurs, make a large range of video buffer amplifers, most good to waaaaay beyond 20MHz, but limited to only a couple of volts output swing (like video levels). Use the older ones which are still available in DIP packages, much easier to use than the SMD-only ones. Look at most of the video buffer app notes, and they show the amp configured as gain of 2, with a 75ohm output resistor, why do they do that? I have used the Texas THS3091 to get +/- 10v at 20MHz+ in a product, but you'd probably have to buy that. I used 3, each with a 150ohm series output resistor, in parallel, to get a very well-defined 50 ohm output impedance. Now why did I do that?

If you do make one of these buffers and use its low impedance output routinely without understanding the consequences of low versus high impedance, the effects of arbitrary loads and cables, then you will get yourself into all sorts of trouble, infintely more than rethinking your experiments for 50 ohms.

Sulaiman

Wed Oct 07 2009, 11:04AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

For this application I use a small transformer,
e.g. a 10:1 transformer will reduce the sig.gen. impedance by 10^2 = 100
so 35 ohms would be transformed to 0.35 ohms etc.
Wind the transformer on a ferrite toroid, wind as many turns as will fit in a single layer,
the secondary (output) can be one or more turns.

The output voltage will be lower, but with a series resonant load it will be multiplied by the circuit 'Q'.

Steve Conner

Wed Oct 07 2009, 11:09AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Terry Fritz designed an amplifier for just this purpose, based on some crazy power op-amp in a TO-3 package. Can't remember the part number, but it's quite a beast, you can get up to 10 amps from it.

I've used Sulaiman's transformer trick (a CT backwards works nicely!) as well as one of those old Maplin MOSFET audio amp boards, running off two bench power supplies. I was using the latter to measure ESR of electrolytic caps by hitting them with a square wave of several amps.

We already explained to you how to measure low-impedance tuned circuits by shunt feeding them. In this case the high impedance of your generator is an advantage.

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