Voltage Follower not outputting the input + voltage

Thread Starter

aomahony

Joined Jan 23, 2020
11
Hey guys,

I've tossed together a circuit to mess around with dynamic CMOS logic...however, when I pass the output into a voltage follower (+ terminal, - terminal is feedback), it works fine when the output is high on the low clock cycle, but on the high clock cycle, the NAND gate "evaluates" to a 0, and the output voltage reduces at the + terminal, but not at the - terminal. I thought that the op-amp amplified based on the difference of voltage, so when the + voltage quickly reduces, the feedback to the - terminal would be negative?

Here is the circuit: http://tinyurl.com/uod8vt7

Thanks!
 

Alec_t

Joined Sep 17, 2013
10,906
Read the datasheet for the 741 op-amp that you are using. In particular note the supply voltage requirements and the common mode input voltage range.
 

Thread Starter

aomahony

Joined Jan 23, 2020
11
Hi @Alec_t ,

I had a look, and it doesn't seem like I'm out of any sort of range...max supply voltage is +-22 V and I can't see anything about the common mode input voltage range...but from my circuit, at the point of capacitor discharge, it will vary about -2.5V. I changed my voltage source to the "ideal" value of 15V and am still getting the issue.
 

Thread Starter

aomahony

Joined Jan 23, 2020
11
I think I got it...

I read on another forum that op-amps often need a reference to negative VDD to work properly with feedback. I tossed in a reference to -5V and it worked!

http://tinyurl.com/tftqp6c

I think I see why: when the non-inverting input drops, the inverting input is still high, so the op-amp will output a negative voltage on the feedback loop. However...the op-amp can't "understand" negative voltage without a reference to it.
 

WBahn

Joined Mar 31, 2012
25,231
Hi @Alec_t ,

I had a look, and it doesn't seem like I'm out of any sort of range...max supply voltage is +-22 V and I can't see anything about the common mode input voltage range...but from my circuit, at the point of capacitor discharge, it will vary about -2.5V. I changed my voltage source to the "ideal" value of 15V and am still getting the issue.
Different data sheets from different manufacturer present the information in different ways. Look at the output voltage swing and you will see that the data sheet reveals that the output voltage doesn't go all of the way to the power supply voltages, but (depending on the load) stops two or three volts short of it. If you look at the functional schematic of the internal workings of the part you will see that the best it could possibly do, even with no load, is get within one diode drop of the supply voltages.

I think I got it...

I read on another forum that op-amps often need a reference to negative VDD to work properly with feedback. I tossed in a reference to -5V and it worked!

http://tinyurl.com/tftqp6c

I think I see why: when the non-inverting input drops, the inverting input is still high, so the op-amp will output a negative voltage on the feedback loop. However...the op-amp can't "understand" negative voltage without a reference to it.
It has nothing to do with negative voltages, but merely with the limitations one the output voltage range of the op amp relative to its power supply rails.
 

crutschow

Joined Mar 14, 2008
24,334
I can't see anything about the common mode input voltage range
The 741 data sheet calls it "Input voltage range" (below).
It's shown for power supplies of ±15V where its minimum value is ±12V.
This means the input can be no higher than 3V below the positive rail, and no lower than 3V above the negative rail.
Thus if the negative rail is ground, the minimum input voltage is +3V.

1582645676801.png
 

AnalogKid

Joined Aug 1, 2013
8,305
I had a look, and it doesn't seem like I'm out of any sort of range...max supply voltage is +-22 V
And the *minimum* operating voltage is ... ?

I don't think the 741 deserves much of the grief it gets on various fora, but - it is uniquely BAD as a logic interface component.

ak
 

Thread Starter

aomahony

Joined Jan 23, 2020
11
The 741 data sheet calls it "Input voltage range" (below).
It's shown for power supplies of ±15V where its minimum value is ±12V.
This means the input can be no higher than 3V below the positive rail, and no lower than 3V above the negative rail.
Thus if the negative rail is ground, the minimum input voltage is +3V.

View attachment 199946
Wait, so when it's positive rail is 5V, the input has to be 2 V?
 
Almost all spec's for a 741 opamp are shown when it has a +/-15V supply (a total of 30V) except two:
1) Its open-loop gain is shown dropping when its supply is as low as +/-5V (a total of 10V).
2) Its maximum peak-to peak output is shown at only 4V when its supply is as low as +/-5V (a total of 10V).

So its minimum supply when it has very poor performance is +/-5V which can be +10V and 0V if its inputs are biased at about +5V, but then its inputs must be between +3V and +7V.
 

crutschow

Joined Mar 14, 2008
24,334
so when it's positive rail is 5V, the input has to be 2 V?
It can be no higher that 2V from the positive rail.
And if you have a -5V negative rail, then the input can go no lower the -2V.

Obviously if you use just a single +5V rail, the op amp won't work.
 
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