How to apply DC amp bias without affecting the output result

[tcwong3]

DC Amp Biasing

I need to amplify dc input voltage (ranges from 0V to 600mV) by 5 times.
Only single supply of 12V is available for my circuit.

I use op amp to form non-inverting amplifier.
It work well when the input is above 0V.
But when the input is 0V, the op amp has no bias current flow into its
non-inverting pin, the circuit become not workable. The output is almost 12V (actual value should be 0V x 5 = 0V) when Vin = 0V.

If I try to bias the input, the bias voltage will be amplified and output become not acurate.

can anybody tell me how do apply bias without affect the output result.

 

[guamak_menanak]

Re: DC Amp Biasing

First of all, did you mean your opamp design is connected to Vdd=12V and Gnd? The main reason why your design is failed is due to some transistor is in the off region. Eventhough you can get some result when you decrease the input near to 0V, maybe some of transistors in the linear-region! We don't want transistor in linear!!

If you need to bias the input, make sure your opamp itself include the bias when you design your opamp! If you just add bias in the input, your opamp maybe will not function as expected!

My suggestion is make sure all the transistors are in saturation in your simulation at every input voltage.

 

[tcwong3]

Re: DC Amp Biasing

Hi! Guamak_menanak, thanks for your reply.

How do add bias to both non-inverting pins in the attached file?

Again thanks.

 

[guamak_menanak]

Re: DC Amp Biasing

That's your circuit?
I thought you are designing in the transisitor level!
I'm not so sure to help you...but in my opinion, you can't add any bias point for that circuit as it can change you main circuit!

Can anybody help tcwong3?

 

[tcwong3]

Re: DC Amp Biasing

This circuit is a very common instrumental amp configuration. It is usually connected to bridge and getting bias from the bridge.

My problem is that I don't use bridge and both of the inputs will sometime be left open. The output must be zero volt in this condition.

I just remember something, the 3 1/2 & 4 1/2 digit ADC IC's that used in the low cost meters can operate with the input pin open (unbiased). Why?