# degeneration differential pair

1. ## degeneration differential pair

Hello

I am designing opam which should read large differential input signal,
as we know from theory that VID= Vov1-Vov2, where Vov is the overdrive voltage for both differential pare transistors.

I tried my best to increase this difference by increasing the tail current or by reducing the transistor size but still the result is limited,

is there better approach?

2. ## Re: degeneration differential pair

"Happiness consists of wanting the right things"

Op amps don't "read" large signals. An ideal, and even
a run-of-the-mill op amp, will have full output swing
on a minuscule input difference. Like, an 80dB AVOL
amplifier with a +/-10V output range will swing across
that range with -1 to +1mV (Vio aside).

Don't know what you plan to do with the excess, but
there's not much you can (unless you want less, way
less, gain - such as operating in closed loop feedback).

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3. ## Re: degeneration differential pair

Hello freebird

Actually I want to use it as instrumentation amplifier in which I read sensor signals from bridge, this signals are changing in the value of one volt.

I am using differential deference amplifier but i called opamp.

because I dont want to use the classical Instrumentation amplifier which consist of three op amp I am designing current sensing InAmp , and for this I am facing the trouble

4. ## Re: degeneration differential pair

You can create a simple differential detector by connecting the sources to each other. Each serves as ground for the other source (in a manner of speaking).

The anti-parallel led's tell which source is greater voltage. Or else measure voltage across the resistor, which allows you to calculate Amperes.

This is only a basic concept. You can add components to achieve filtering, level-shifting, waveform conditioning, etc.

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5. ## Re: degeneration differential pair

Thank you Red,

I think adding level shifter is an good idea,,, such a circuit of source follower which shift down the input signal to VGs-Vth,

wha about degenrative source, what the are used for?

6. ## Re: degeneration differential pair

Source (or emitter) degeneration is often used for a
tradeoff between DC gain and slew rate. But DC gain is
not knocked down to the level you are after.

You do know there are many perfectly fine 8-pin
instrumentation amplifiers on the market, for about a

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7. ## Re: degeneration differential pair

thank you again freebird

the thing is that my task to design the inamp

the signals coming from the bridge are +-1V while my amplifier accept +-0.6 V,,,, is it possible to use the source follower as a level shifter ? or this idea doesnt work

8. ## Re: degeneration differential pair

Let's start by distinguishing between the range where you
want proper function, and the range where you need to
guarantee survival.

Then, your desired gain of the difference signal, and the
output signal swing (range).

a resistor divider (pair), throwing away 50% of signal up
front could fix the whole thing?). Presuming that the source
can't itself be convinced to reduce its swing, and that the
entire range has useful information to be processed.

I think I've seen papers on making instrumentation amps
with less than three amplifiers but can't recall where, or the
suggestions. Also 3-amp InAmps can have problems in low
supply voltage circuits and high signal, headroom-wise.

Do you consider the possibility that I/O devices and a
higher working rail could also be a solution, post-scale
if needed? Or is there only one, low voltage supply?

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9. ## Re: degeneration differential pair

i am using current feedback InAmp,,,

do you mean that if i shift the signal i will lose the information ? may be i didnt understand this point

10. ## Re: degeneration differential pair

Originally Posted by Junus2012
the signals coming from the bridge are +-1V while my amplifier accept +-0.6 V,,,, is it possible to use the source follower as a level shifter ? or this idea doesnt work
Why don't you use your rail-to-rail amplifier fed back for a very low gain, say 2..3 , depending on your supply voltage, as dick_freebird suggested above (last line)?

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