I am back on adjustable negative power supply using operational amplifier.

First, I am sorry we really have some reservations on component selection. I understand there are other operational amplifiers with large output current and some are cheaper too but we are limited to use the operational amplifier AD8397 and the PNP and NPN as shown in the attached schematic.

Regarding output load impedance and load characteristics, there is nothing much information given for now. I just use 1k Ohm as load in simulation. We actually need to change the negative voltage across the load very slowly. This can be with a pause of at least one second or more.

I am considering operational amplifier based solution for adjustable negative power supply. There is a change in the input voltage to the operational amplifier. The input voltage to the operational amplifier AD8397 is supplied by the digital to analog DAC which is interfaced to the FPGA through I2C. The digital to analog DAC can have maximum of 2.5 V. This device is again selected. We can have 0 to 2.5 V out from DAC which is connected to operational amplifier's inverting input.

The operational amplifier is inverting the input and with some gain and using push-pull BJT booster stage can have 0 to -3.3 V across the load. I have simulated the attached circuit using sine wave full swing at the input on the operational amplifier, just to see the output.

I have some questions.
1- There has to be a feedback resistor for stability. I am not sure if I am getting it completely. Kindly explain bit more where can I put the feedback resistor.

2- The push-pull class AB stage have cross-over issue which can be resolved by two diodes or biasing the gates of both BJTs in such a way that they always conduct. It's not an issue if we don't solve the cross-over issue completely. We don't need full swing anyways.

3- There are gain margin and the phase margin related issues. I need some help here too. Kindly have a look at the attached circuit and let me know how can I address them along with the stability.

4- How clean would be this adjustable negative power supply using operational amplifier and push-pull BJT booster stage, compared to SMPS and linear regulators.

 

I am back on adjustable negative power supply using operational amplifier.

First, I am sorry we really have some reservations on component selection. I understand there are other operational amplifiers with large output current and some are cheaper too but we are limited to use the operational amplifier AD8397 and the PNP and NPN as shown in the attached schematic.

Regarding output load impedance and load characteristics, there is nothing much information given for now. I just use 1k Ohm as load in simulation. We actually need to change the negative voltage across the load very slowly. This can be with a pause of at least one second or more.

I am considering operational amplifier based solution for adjustable negative power supply. There is a change in the input voltage to the operational amplifier. The input voltage to the operational amplifier AD8397 is supplied by the digital to analog DAC which is interfaced to the FPGA through I2C. The digital to analog DAC can have maximum of 2.5 V. This device is again selected. We can have 0 to 2.5 V out from DAC which is connected to operational amplifier's inverting input.

The operational amplifier is inverting the input and with some gain and using push-pull BJT booster stage can have 0 to -3.3 V across the load. I have simulated the attached circuit using sine wave full swing at the input on the operational amplifier, just to see the output.

I have some questions.
1- There has to be a feedback resistor for stability. I am not sure if I am getting it completely. Kindly explain bit more where can I put the feedback resistor.

2- The push-pull class AB stage have cross-over issue which can be resolved by two diodes or biasing the gates of both BJTs in such a way that they always conduct. It's not an issue if we don't solve the cross-over issue completely. We don't need full swing anyways.

3- There are gain margin and the phase margin related issues. I need some help here too. Kindly have a look at the attached circuit and let me know how can I address them along with the stability.

4- How clean would be this adjustable negative power supply using operational amplifier and push-pull BJT booster stage, compared to SMPS and linear regulators.

Try connecting R6 to the common point of the 2 output emitters. You can also reduce R1 if you need more base drive.

 

You mean R6 between Vin (-ve) and the common point of emitters. I guess you mean by the common point, the point connected/driving the base of both BJT transistors. In the attached, P1 and P2 are marked. The R6 should be connected to P2 instead P1.

I will also try to reduce the R1. But this I think will increase the base current Ib.

PS: I am sorry. There should be a new post from # 21.

 

You mean R6 between Vin (-ve) and the common point of emitters. I guess you mean by the common point, the point connected/driving the base of both BJT transistors. In the attached, P1 and P2 are marked. The R6 should be connected to P2 instead P1.

I will also try to reduce the R1. But this I think will increase the base current Ib.

PS: I am sorry. There should be a new post from # 21.

No, I meant remove R6 from point P1 and re-attach it to the output point where the 2 emitters come together.
As for reducing R1, yes, this will allow the opamp to supply more drive current to the bases, if they require it, depending on the current that your load takes. Try it "as is" for now!

 

No, I meant remove R6 from point P1 and re-attach it to the output point where the 2 emitters come together.
As for reducing R1, yes, this will allow the opamp to supply more drive current to the bases, if they require it, depending on the current that your load takes. Try it "as is" for now!

I understand this. I will run simulation with these changes.

 

Now I attached R6 to the output point where two emitters come together.
I also change the value of R1 from 1k ohm to 500 ohm. I guess this will increase the base current.
The resistor values R6 and R3 are changed to get 0 to -3.3 V at the Vout.
Two diodes are added to remove cross-over.

 

Now I attached R6 to the output point where two emitters come together.
I also change the value of R1 from 1k ohm to 500 ohm. I guess this will increase the base current.
The resistor values R6 and R3 are changed to get 0 to -3.3 V at the Vout.
Two diodes are added to remove cross-over.

The two diodes aren't actually doing anything because they are shorted out. You can either remove them, and reconnect the two bases as they were before, or connect R1 to the common point between the two diodes.
However, if you do that, there is the possibility of excessive bias between the bases of the two transistors allowing a large current to flow through both transistors from the positive supply to ground! This will happen if the voltage drop across the two diodes is greater than the voltage drop across the two base/emitter junctions. This is usually alleviated by adding a pair of low value resistors to the emitters of the transistors to provide stabilizing feedback!
With your circuit as it stands, the massive opamp gain overcomes the crossover switching points of the base/emitter voltages, and you never see it!