I'm trying to increase my knowledge of bootstrapping techniques and started experimenting with a simple bootstrapped op amp. It's all done in LTSpice so far. I can get it to work just fine, however... It's extremely finicky on component values. The schematic below works fine, but if I change R3 and R7 from 15k to 12k, the whole thing latches up. I'm suspecting this has to do with SPICE. Maybe it's having trouble finding the OP with all the loops in the circuit?
Here's the performance with 15k resistors. Output in the middle and high and opamp VCC and VEE at top and bottom. Looks great!
Now I nudge the values of R3 and R7 to 12k. As you can see, it latches almost immediately. Is this a legit design problem or SPICE weirdness? BTW, increasing the resistors to 18k has the same effect. I tried replacing them with zeners and voltage sources, but to no avail.
Here's the performance with 15k resistors. Output in the middle and high and opamp VCC and VEE at top and bottom. Looks great!
Now I nudge the values of R3 and R7 to 12k. As you can see, it latches almost immediately. Is this a legit design problem or SPICE weirdness? BTW, increasing the resistors to 18k has the same effect. I tried replacing them with zeners and voltage sources, but to no avail.
The usual first step in troubleshooting problems like this, is to plot the currents and the power dissipations. Then study them carefully and look for surprising or nonsensical values.
- Plot Ic, Ib, Ie of Q1
- Plot Ic, Ib, Ie of Q2
- Plot supply+ current of U1. Plot supply- current of U1
- Plot current flowing into Vin+ , plot current flowing into Vin- , plot current flowing into/out of Vout of U1
Did you check current from op-amp out, base currents of transistors, voltage at transistor bases?
Have a look at elvee's ez-dump!
Edit: also keep in mind common mode input range. Ne5532 does not like input above/below supply.
In your example it seems positive supply gets below -2 V. At this point you probably leave common mode range.
Have a look at elvee's ez-dump!
Edit: also keep in mind common mode input range. Ne5532 does not like input above/below supply.
In your example it seems positive supply gets below -2 V. At this point you probably leave common mode range.
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- if you upload the *.asc file it would be easier to help you
- regarding the latch-up: did you check the supply voltage (VCC and VSS) of your opamp? What is it with R3|7==15k and what it becomes with 12k?
That seems like a likely explanation. It looks like it finds a metastable state where VCC goes negative (about -3.6V). Check it out:
It can be negative without problems, but the inputs should never be higher than supply (minus some overhead).
You could try lowering the gain, just to test it.
In that case inputs follow output more closely.