Kevin Weddle
- Feb 23, 2004
- 1,620
- Joined
- Feb 23, 2004
- Messages
- 1,620
audioguru2
- Apr 6, 2004
- 12,026
- Joined
- Apr 6, 2004
- Messages
- 12,026
Kevin, you need to learn about negative feedback.
Your simple voltage regulator has no voltage gain. Its voltage gain is a little less than 1.
Its 200k resistor does almost nothing (it reduces the voltage gain from exactly 1 to a little less than 1).
The opamp is pretty fast but the darlington connected transistors are probably pretty slow causing phase shift and causing the circuit to oscillate.
View attachment 42506
Your simple voltage regulator has no voltage gain. Its voltage gain is a little less than 1.
Its 200k resistor does almost nothing (it reduces the voltage gain from exactly 1 to a little less than 1).
The opamp is pretty fast but the darlington connected transistors are probably pretty slow causing phase shift and causing the circuit to oscillate.
View attachment 42506
Kevin Weddle
- Feb 23, 2004
- 1,620
- Joined
- Feb 23, 2004
- Messages
- 1,620
The schematic shown has a gain of 10. I changed the gain to 10,000 in this circuit. I don't know how unstable that would make it. I guess I'll have to match the opamp and output transistors.
Last edited by a moderator:
It appears as though you've confused the non-inverting and inverting amplifier topologies.
As we said many times in the thread linked to above, you need to build a non-inverting amplifier with the negative feedback taken from the output side of the buffer.
Read the documents linked below. If you don't understand them, read them again. Then if it's still not clear ask here.
https://www.onsemi.com/pub/Collateral/SR004AN-D.PDF
http://www.ti.com/lit/an/snva558/snva558.pdf
As we said many times in the thread linked to above, you need to build a non-inverting amplifier with the negative feedback taken from the output side of the buffer.
Read the documents linked below. If you don't understand them, read them again. Then if it's still not clear ask here.
https://www.onsemi.com/pub/Collateral/SR004AN-D.PDF
http://www.ti.com/lit/an/snva558/snva558.pdf
Kevin, if you are referring to your first circuit in this thread, it cannot work.
The Vbe of the Darlington will always be around 1.5V so the voltage across the 20K resistor will be about 135mV.
Ignoring the input offset, the two input voltages of the op-amp must be the same, so the output voltage will therefore settle at Vset minus 135mV, and there is no way you can get it any higher.
If you change the values of the two feedback resistors to alter its gain, the output voltage will lie somewhere between Vset and Vset minus Vbe, and as Vbe varies with load, then so will the output voltage.
The Vbe of the Darlington will always be around 1.5V so the voltage across the 20K resistor will be about 135mV.
Ignoring the input offset, the two input voltages of the op-amp must be the same, so the output voltage will therefore settle at Vset minus 135mV, and there is no way you can get it any higher.
If you change the values of the two feedback resistors to alter its gain, the output voltage will lie somewhere between Vset and Vset minus Vbe, and as Vbe varies with load, then so will the output voltage.
Last edited by a moderator:
Kevin Weddle
- Feb 23, 2004
- 1,620
- Joined
- Feb 23, 2004
- Messages
- 1,620
I went ahead and built this circuit today. The input resistor voltage changed very little. The output transistor voltages changed quite a bit.
audioguru2
- Apr 6, 2004
- 12,026
- Joined
- Apr 6, 2004
- Messages
- 12,026
You make no sense because your circuit has no input resistor. Its input is the (+) input of the opamp.KevinIV said:
Similar threads
