OP-Amp issues - what am I doing wrong?

[Garry2009]

OP-Amp issues...

Hello all...I'm a new member here:

I've got a DAC circuit with 0-3v Vout, and I need to amplify that to 0-5v for input to a high voltage op-amp circuit.

I'm using a simple 741 op-amp (non-inverted) circuit for this...560-ohms across -Input and Vout (Pin-2 and Pin-6), and 1K ohms from -Input (Pin-2) to ground.

With 3.0v on +Input I get about 4.78v on Vout, which is okay...but with just a few mV on the +Input (Pin-3) I get 3.0v out.

Something seems to be floating, but I don't know what...I was hoping some of you guys might know what I'm doing wrong (or what I need to do right

Thanks in advance,
Garry

 

[courcirc8]

Re: OP-Amp issues...

Hmm, you don't describe completely your design...
What is the supply? +-15V? If you use for example only +10V (0V on ground) you will saturate the output for low input voltages.

 

[Garry2009]

Re: OP-Amp issues...

Thanks for the reply...

Okay, if it works, the circuit is attached here.

The supply is a +12v supply (don't have 15v anywhere in the system), and reference is to ground.

Am I missing something? Seems like "0" in should be "0" out...

Garry

 

[BrianSu]

OP-Amp issues...

In my think, the cause reason for your problem 0V input then get around 3V output. It's u connect 1k ohm resistor to -5V not 0V.
741 is amp dual supply opamp op at +5V and -5V.

 

[bongkk]

OP-Amp issues...

You have configured it as a "comparator". In your case pin 3 should be connected to gnd. Your input should be on pin 2. Discoonect the lower end of the 1 k resistor from the gnd and use it as the input. Note that the gain is roughly .56 (560/1000). You need a gain of 5/3 or 1.67, so replace the 560R resistor with a 1.67K. Instead use a 2K variable to adjust the gain.

 

[Garry2009]

Re: OP-Amp issues...

Thanks everyone for the comments:

BONGKK: Okay, that makes sense. So what about Pin-4 (-Supply)...does that still go to gnd?

Garry

 

[Audioguru]

OP-Amp issues...

A lousy old 741 opamp needs to have an additional supply: a negative supply, for its output to go down to ground.

You could use an MC33171 or MC34071 single-supply opamp instead and when its input is 0V then its output is very close to 0V.

 

[Garry2009]

Re: OP-Amp issues...

Hey, Audioguru:

Thanks! yeah, I've already figured out that the 741 ain't gonna cut the mustard for my application. Are you familiar those chips (MC33171 / 34071)? My DAC operates at 66 Khz - will these guys handle that speed? As I said, the DAC Vout is 0-3.3v and I need to amplify it to 0-5v. If these things will work at 66 Khz, do you possibly have a circuit diagram laying around that I could refer to? I'm handy enough to build something...not genious enough to design it.

Thanks for your suggestion...
Garry

 

[Audioguru]

OP-Amp issues...

The lousy old LM358 dual opamp and LM324 quad opamp are low power, have crossover distortion and go up to 2kHz. But the inputs work at the negative supply voltage that can be ground, the output goes almost to the negative supply voltage and the minimum supply is 3V.

The MC33171 single, MC33172 dual and MC33174 quad opamps are also low power, their inputs also work at the negative supply voltage, the output also goes almost to the negative supply voltage and the minimum supply is also 3V. They do not have crossover distortion and they go up to 35kHz.

The MC34071 single, MC34072 dual and MC34074 quad opamps have normal supply current, their inputs also work at the negative supply voltage, the output also goes almost to the negative supply voltage and the minimum supply is also 3V. They do not have crossover distortion and they go up to 100kHz.

The lousy old 741 opamp does not have anything good about it. It goes up to 9kHz and works poorly when its dual supply voltages are less than about plus and minus 6V.

 

[dipnirvana]

Re: OP-Amp issues...

use AD810 or AD8510 , this good at that speed and signle supply too