Author Topic: Problem (?) with simple op amp circuit  (Read 4709 times)

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Offline AxleD

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Problem (?) with simple op amp circuit
« on: May 24, 2015, 02:41:13 pm »
Hi,

I have built the attached circuit on a breadboard but the result is very different from that simulated in spice. In spice the simulation shows Vout tracking the change in V1.

In the circuit i put together, the only change i have made from the attached schematic is that I am using a 741 opamp.
Now if i select 5.8V at V1, I get around 6.8V at vout (instead of the 5.8V expected).

This circuit also seems to like to destroy opamps and transistors. It is not clear why the circuit is not working as modeled or why things are blowing up (2 opamps, 3 transistors so far). Any ideas on what the issue could be?

Thanks
 

Offline Andy Watson

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Re: Problem (?) with simple op amp circuit
« Reply #1 on: May 24, 2015, 02:47:43 pm »
Any ideas on what the issue could be?
Is it stable? The configuration of Q1 and Q2 introduces extra gain into the loop. Try reconfiguring it using a npn for Q2 as an emitter follower.
 

Offline Asmyldof

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Re: Problem (?) with simple op amp circuit
« Reply #2 on: May 24, 2015, 03:07:07 pm »
You are forward biasing a transistor in your feedback loop. That is possible, but it does require some care and attention to prevent hinky oscillations, something like those oscillations might be a cause for dead op-amps.

Another cause could be current drain at the output in case of sudden input voltage increases, though I'd expect that to be of such short duration that it wouldn't cost you many op-amps in a single go.

A reason for Q2 to break would be high dissipation, 6.8V across 100ohm is 68mA. if your supply really is 20-ish volts, that means the transistor is "eating" about 13.2V, because, where else does it go? That means it dissipates 13.2V * 68mA = 890mW.
If it's a normal TO92 type that'll very likely kill it, even soldered into a PCB, when working in a breadboard (are you using one?), it's anybody's guess. (Because the heatflow in a soldered situation can be predicted, in a breadboard that's significantly harder).

As a side interest, if you've destroyed one part, you might want to change the measurement points and types a little and try again, but when you've destroyed two parts in the same set-up, that's usually the point to critically examine the construction and/or schematic.

Q1 could be having problems with base currents in case of oscillation or spikes on the input, but with a 741 it seems that one is reasonably safe.
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Offline Andy Watson

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Re: Problem (?) with simple op amp circuit
« Reply #3 on: May 24, 2015, 03:23:34 pm »
Q1 might be dying due to its base-emitter being reverse biased - the B-E junction is often only good for a few volts. 
 

Offline Mark Hennessy

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Re: Problem (?) with simple op amp circuit
« Reply #4 on: May 24, 2015, 03:43:10 pm »
At the very least you need a resistor between the output of the op-amp and the base of the first transistor. Though what are you trying to achieve here? If you want the op-amp to provide more current capability, then use transistors configured as emitter followers rather than voltage amplifiers.
 

Offline c4757p

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Re: Problem (?) with simple op amp circuit
« Reply #5 on: May 24, 2015, 03:57:11 pm »
At the very least you need a resistor between the output of the op-amp and the base of the first transistor.

For an emitter follower? A small one might be good to prevent parasitic oscillations, but you hardly need it, unless I'm really missing something here...
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Offline Mark Hennessy

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Re: Problem (?) with simple op amp circuit
« Reply #6 on: May 24, 2015, 04:10:47 pm »
At the very least you need a resistor between the output of the op-amp and the base of the first transistor.

For an emitter follower? A small one might be good to prevent parasitic oscillations, but you hardly need it, unless I'm really missing something here...

It's not an EF  :)
 

Offline AxleD

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Re: Problem (?) with simple op amp circuit
« Reply #7 on: May 24, 2015, 07:34:28 pm »
Thanks that makes sense and i will check for stability and if i am exceeding the specifications of any components.

This will be part of a larger circuit.
The idea here was to control a pass transistor (TIP32C) with two op amps to give voltage and current control for a power supply (as in the attached).

The output seems stable and i do have voltage control - the problem is the output is very different to the modeled output in spice (the spice graph in the attachment is Vout). Q2 was originally intended to be a TIP32C and i used the 3906 just to test the circuit. If i replaced the 3906 with a TIP32C, then I get Vout = 10V instead of the 5.8V (selected voltage at V1) expected!

Just struggling to see why the large difference between the modeled outcome and what I am getting. There is obviously an issue with this design and any ideas on how best to control one transistor with two opamps would also be helpful?
 

Offline Waff0l

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Re: Problem (?) with simple op amp circuit
« Reply #8 on: May 24, 2015, 07:42:06 pm »
Putting in base stopper resistors will help with not destroying too many transistors.
 

Offline dacman

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Re: Problem (?) with simple op amp circuit
« Reply #9 on: May 24, 2015, 08:58:22 pm »
The LT1013 has much greater input impedance.

LT1013 has much more gain.

Looking at the datasheet, I would not drive less than 2 kOhm with the 741 (600 Ohm for the LT1013).

LT1013 can use one supply (-15V supply not needed, but it is for the 741).

Edited to add: If you are using  +21 and -15 Vdc for +Vcc and -Vcc, then the Op Amp is effectively using +/- 18 Vdc, because its specified voltages are referenced to the midpoint of the supply voltages.
« Last Edit: May 25, 2015, 05:26:31 am by dacman »
 

Offline AxleD

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Re: Problem (?) with simple op amp circuit
« Reply #10 on: May 26, 2015, 05:42:42 am »
Thanks.

After adding a resistor to the output of the 741 and from the emitter to ground (in the NPN transistor), the circuit is behaving. Output is now tracking the input. It is still 10-15mv out, but that can be explained by the specific characteristics of the 741 op amp.

To try and improve this i swapped the 741 for the LT1013 running off +/-15V. As my luck would have it, the circuit doesn't work again. I noticed two issues:

1. The V+ input remains at 11.5V "overriding" the reference voltage i select and
2. the Opamp is getting a little warm.

The only thing i think of is that there is some oscillation or the opamps are bad? Any ideas on what i should be looking for to sort this issue out?

Just to clarify on point 1, it looks like the V+ input is loading down the resistor divider that provides the reference voltage.


« Last Edit: May 26, 2015, 06:25:03 am by AxleD »
 

Offline Andy Watson

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Re: Problem (?) with simple op amp circuit
« Reply #11 on: May 26, 2015, 09:41:07 am »
How have you connected the other amplifier in the chip? Have a look at page 10 of the datasheet and make sure that you haven't violated any of the conditions that cause phase reversal.
 

Offline AxleD

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Re: Problem (?) with simple op amp circuit
« Reply #12 on: May 26, 2015, 11:58:54 am »
The other amplifier is not connected. Just floating.

Double checked and the bugger is oscillating. Pretty sine wave 500 mv peak to peak. Given the info from the datasheet, this could definitely be the issue. Not sure how it would affect the inputs though.

Will do some reading on the matter as I have no clue how to sort this out or where it coming from.

Thanks to all for the input.
 

Offline atferrari

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Re: Problem (?) with simple op amp circuit
« Reply #13 on: May 26, 2015, 12:12:10 pm »
When using modern opamps (vis-a-vis the 741) like the LT1013 you could go away by using lower voltages. Even the input signal (15V is quite high just for this kind of test). You well could be using just 0,5 or 1 V at the input. Be friendly with the op amp.

I always try things with +/- 9V supplies. You are using 21+15 = 36V for the test.

Anyhting wrong will mean lot of current going somewhere.

Base resistor is part of the game. (I had to remind that to myself few days ago). See my post elsewhere about an opamp sucking too much current in a CCS.

In LTSpice, labels are good to unclutter the schematic and make changes easily. Learnt that part too.

Buena suerte.
« Last Edit: May 26, 2015, 07:22:13 pm by atferrari »
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Offline Andy Watson

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Re: Problem (?) with simple op amp circuit
« Reply #14 on: May 26, 2015, 03:03:08 pm »
The other amplifier is not connected. Just floating.
I would stop it floating before worrying about anything else.
 

Offline dacman

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Re: Problem (?) with simple op amp circuit
« Reply #15 on: May 27, 2015, 02:47:15 am »
If I plug the OP27 (from LTSpice) into the original circuit, the circuit oscillates (in LTSpice).
« Last Edit: May 27, 2015, 04:27:13 am by dacman »
 

Offline LvW

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Re: Problem (?) with simple op amp circuit
« Reply #16 on: May 31, 2015, 11:17:13 am »
If I plug the OP27 (from LTSpice) into the original circuit, the circuit oscillates (in LTSpice).

The overall feedback factor is larger that unity - caused by the transistors extra gain. Such a circuitry is always unstable or - at least - it tends to be unstable (poor margin) .
In principle, this applies to ALL opamps.
For my opinion, the only solution consists in introducing one of the classical methods of external phase compensation (at the input of the opamp).

Edit: If allowed - you should decrease the gain of the transistor circuitry (in addition to compensation measures).
« Last Edit: May 31, 2015, 01:55:33 pm by LvW »
 

Offline AxleD

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Re: Problem (?) with simple op amp circuit
« Reply #17 on: June 01, 2015, 05:27:59 am »
Thanks.
Have not had a chance yet but will get back to testing out and investigating the suggestions.

As an aside, there is a strong possibility that the LT1013's i bought are damaged.
I put together a simple circuit powering the op amp from a 9v battery and it still didn't work. The opamp is still "loading down" the divider that creates the reference voltage.
Tried a 358/258/741/TLC227 and they all work in same configuration.

Cheers
« Last Edit: June 01, 2015, 05:47:57 am by AxleD »
 


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