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Om amp power supply voltage

by unknown_2
Tags: power, supply, voltage
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unknown_2
#1
Oct9-10, 12:33 PM
P: 29
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.
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wclawson
#2
Oct9-10, 12:41 PM
P: 17
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
#3
Oct9-10, 01:03 PM
P: 4,663
What is your output circuit?

Bob S

unknown_2
#4
Oct9-10, 01:06 PM
P: 29
Om amp power supply voltage

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
#5
Oct9-10, 01:07 PM
P: 29
looking for a way to calculate it so i can apply it to different Vcc voltages
Bob S
#6
Oct9-10, 01:30 PM
P: 4,663
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
#7
Oct9-10, 01:39 PM
P: 29
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
#8
Oct9-10, 02:45 PM
P: 4,663
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
#9
Oct9-10, 04:38 PM
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P: 12,064
Quote Quote by unknown_2 View Post
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.
I don't know if all datasheets follow the same convention, but for the TL08x series, TI uses VOM 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 VOM, 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?

Quote Quote by unknown_2 View Post
looking for a way to calculate it so i can apply it to different Vcc voltages
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
#10
Oct9-10, 04:45 PM
P: 29
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.


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