# multistage op amp design analysis

#### katy1163

Joined Apr 26, 2018
8
Can anyone give me a clue as to how to start the analysis of the attached circuit please? The feedback loop to the voltage follower is throwing me off....

#### ericgibbs

Joined Jan 29, 2010
11,556
hi katy,
Welcome to AAC.
Is this a college assignment.?
E

#### katy1163

Joined Apr 26, 2018
8
Nope--this is for a real world application (reading temperature). R10 in this circuit is actually an RTD. This circuit was designed by someone that is no longer working at my company--certainly not how I would have done it, but I have to live with it for now. I'm also terrible at analog electronics!

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#### Hymie

Joined Mar 30, 2018
923
Nope--this is for a real world application (reading temperature). R10 in this circuit is actually an RTD. This circuit was designed by someone that is no longer working at my company--certainly not how I would have done it, but I have to live with it for now. I'm also terrible at analog electronics!
U4 is the output of the circuit configured as a comparator, so the output will be either high or low.

The output of U3 is fed into a resistor chain made up of R7, R10 (the RTD) and R9 to 0V, with R7 & R9 equal in value.

The voltage at R7/R10 is fed back to U3 (via U1).

At a critical value of R10 the output of U3 will change from + voltage to – voltage, changing the comparator output state.
(it is the change of U3s output, to either a positive or negative voltage, that is changing the output comparator state)

I suggest you replace R10 with a low value variable resistor and observe the circuit performance with various values of resistance.

Without analysis the circuit further, I suspect U1/U3 and associated components are providing some hysteresis to the response.

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#### crutschow

Joined Mar 14, 2008
25,983
I can make little sense of that circuit.
As configured, U6's (+) input is always higher than its (-) input for any finite value of R10 so the output will always be high.
I think there's an error in the schematic.

Do you have a schematic of the original circuit, not the LTspice copy?

#### Hymie

Joined Mar 30, 2018
923
I can make little sense of that circuit.
As configured, U6's (+) input is always higher than its (-) input for any finite value of R10 so the output will always be high.
I think there's an error in the schematic.

Do you have a schematic of the original circuit, not the LTspice copy?
Perhaps the low end or R4 should be at –5V and not 0V (or even +5V)

#### ebp

Joined Feb 8, 2018
2,332
Is this copied from a schematic or traced from a circuit board?

There are many very strange things about the circuit - R1 has no useful purpose unless the intent is to force an offset voltage due to amplifier bias current; ditto R3; the open loop gain of the amp is at least 500 000, so the output will swing rail to rail for about 20 microvolts across or 160 nanoamps through R10, which means shifting the output of U3 by bout 200 µV is sufficient to cause the output to swtich; the feedback around the U1 & U3 loop is positive (which might make sense if there were a capacitor in there somewhere to impose some rule on behavior).

#### ebp

Joined Feb 8, 2018
2,332
I can't see any way that the output of U3 doesn't just go to 5 volts (minus headroom required by amp) and stay there, so to rephrase crutschow, this is a complicated circuit to make a steady 5 volts across R5.

#### katy1163

Joined Apr 26, 2018
8
Sorry--Some detail is left out in the spice circuit...Here is a snapshot of the actual schematic. I'm mostly concerned with determining how to find the input to U4.

#### katy1163

Joined Apr 26, 2018
8
Clarification for above post: U4 in Spice circuit, but U22 in schematic picture

#### crutschow

Joined Mar 14, 2008
25,983
Clarification for above post: U4 in Spice circuit, but U22 in schematic picture
I don't see a U22.

#### katy1163

Joined Apr 26, 2018
8
Haha oops--different copy of the same circuit. It's used several times on this board. In that picture, it's U16

#### crutschow

Joined Mar 14, 2008
25,983
And whats the relation between that circuit and the Spice circuit, as they appear quite different?

#### katy1163

Joined Apr 26, 2018
8
That's why I posted the spice one--the schematic one is drawn confusingly. Pretend R10 in the spice circuit is connected between terminals 1 and 2 on J6 and there is a short between 2 and 4 and also between 1 and 5. I think that is all the external detail necessary to complete the functionality....

#### Harley Burton

Joined Jan 1, 2018
12
Am I overlooking U4 in #9?

You have some good people looking at your question (not referring to myself), but you should make your question easy to understand for them. Post a single schematic that contains everything involved in the question, and use that schematic to frame your question, and I bet you'll get an answer.

#### katy1163

Joined Apr 26, 2018
8
Yes I apologize for not being clear enough. The U4 I referred to in #9 is in the Spice circuit that I posted in #1. U4 in Spice pic in #1 = U16 in the schematic in #9.

#### katy1163

Joined Apr 26, 2018
8
Full schematic

#### crutschow

Joined Mar 14, 2008
25,983
Suggest you try to do a simulation of the complete circuit, not just part of it.