Om amp power supply voltage

unknown_2

Hi, i'm trying to understand how to correct when you put Vcc as 10V your max output is like 9.18V. Is there a factor on the data sheet or what? I'm trying to build a bi-state multivibrator to generate a 10V peak value but it's being saturated.


thanks.

wclawson

Vout = A(Vp-Vn)

A = open loop gain specified by op amp (usually around x10^5)
Vp = non-inverting input
Vn = inverting input

Vcc is normally around tens of volts. Have you tried increasing it to change the range?

Based on the eq above, in the linear range:

-Vcc< Vout < -Vcc

Bob S

What is your output circuit?

Bob S

unknown_2

well open loop gain is so big that it'll, ideally saturate at Vcc and -Vcc. but practically it'll saturate at a lower value ( 10V saturate to 9.11V). i want to increase Vcc and -Vcc such that i can compensate for this decrease in output voltage. so in order to saturate at say 10V I need to increase Vcc and -Vcc to a higher number like 10.66V to compensate.

unknown_2

looking for a way to calculate it so i can apply it to different Vcc voltages

Bob S

Why don't you use a voltage comparator with an npn open collector output with a resistor pullup, or a pnp collector output with the emitter tied to Vcc? What circuit are you using?

Bob S

unknown_2

i'm limited to resistors, opamps and capacitors. My full circuit it a waveform generator. the bi-state multivibrator output is acting as a square wave generator. and i want a 8vp-p value. but if i put 4V and -4V for Vcc and -Vcc it'll def be saturated and inaccurate.

Bob S

What you could use is a rail-to-rail-output op amp like the Linear LTC1152 chip in an 8-pin DIP package

http://www.google.com/url?sa=t&sourc...mhjOxOgKsCOx_g

You could also use two bipolar transistors; a pnp pullup to Vcc and a npn pulldown to Vee, with their collectors tied together.

Bob S

Redbelly98

I don't know if all datasheets follow the same convention, but for the TL08x series, TI uses V_OM for the maximum output voltage. Look at Figure 10 on p. 10 at

http://focus.ti.com/lit/ds/symlink/tl081a.pdf

It is a graph of max. peak output voltage vs. supply voltage, under 10 kΩ load. So for that type of op-amp, you'd need at least ±11.2V supplies to get ±10V out.

Also, the specs on p. 7 mention V_OM, the max. peak output voltage swing, is at least ±12V and typically ±13.5V for a load of 10 kΩ and ±15V inputs.

That being said, could you just try supplying ±12V to your op-amp and see what happens?

The graph I referred to above shows a pretty linear response. If your op-amp's datasheet doesn't have such a curve, then you could measure max. output for a few different supply voltages, with whatever load resistance you use in practice, and make your own graph.

unknown_2

what i've done is put 15V Vcc and the max out on the 741 is about 13V. so that 13V output i'll just shrink the amplitude to get my desired value.