Hello!

I have a circuit on my PCB, which has a differential input stage and buffer circuit with transistor at output of the buffer amplifier to draw high current (like around 0.5A) from the supply. The circuit has a problem at the output stage is that the output voltage is not stable, the output voltage is derating around 50mV with load, without load I measured 50mV higher. My target is to maintain the output voltage constant at both with load and without load. Could you suggest me how to achieve this?. See my circuit is attached below.

 

The circuit shown in post #1 does not provide any information as to where the output voltage is located. I could guess that it is the voltage across R3, but that is only a guess. Based on that guess, probably the power supply lacks adequate regulation. OR suffers from inadequate current delivery ability.

 

V2->Q1C->Q1E->R3 path may have large current; Q1 may not work properly. Do you miss another resistor in the path, or the value of R3 is too small?

 

The circuit has a problem at the output stage is that the output voltage is not stable, the output voltage is derating around 50mV with load,

What is the "output stage" and where is the "output" that are you measuring this 50mV change?

Post your .asc simulation file.

 

Hello!

I have a circuit on my PCB, which has a differential input stage and buffer circuit with transistor at output of the buffer amplifier to draw high current (like around 0.5A) from the supply. The circuit has a problem at the output stage is that the output voltage is not stable, the output voltage is derating around 50mV with load, without load I measured 50mV higher. My target is to maintain the output voltage constant at both with load and without load. Could you suggest me how to achieve this?. See my circuit is attached below.

View attachment 349702

To add to the questions, where does the load that you refer to, appear in the circuit? what is the nature of the load? purely resistive?

 

That is 1% regulation, what do you require?

 

I presented a probable cause of the output voltage variation back in post #2! And no response or comment about what is the most likely cause. I am GUESSING that the output voltage is across R3, the resistor in series with the emitter current.

The Thread Starter needs to understand that not everybody has that simulator software to open an .asc file. Show the actual circuit, as built, along with the specifications of the power supply being used.

And understand that I have no intention of having that simulator on any of my computers.

 

Hello!

Yes, the resistor R3 is the load. Yes, the load resistor is small because I need to draw 0.5A from the power supply.

Thank you!

 

hi pp,
As requested, please post your LTS asc file.

E

 

Simulation file attached

 

Does your simulation include resistors to simulate the pcb track and power supply source resistances?

 

Does your simulation include resistors to simulate the pcb track and power supply source resistances?

I haven't consider that

 

The load-dependent output voltage may also be a result of the opamp not being perfect/ideal. Note that the characteristics of that particular opamp are specified assuming a dual-polarity supply. Try a different opamp in the simulation and compare results.

 

50mV error represents the voltage across 10mm of 0.5mm wide pcb track carrying 0.5A. Does that give you a hint as to where to look for the problem?

 

Is "no load" R3 disconnected from the circuit?

 

Is "no load" R3 disconnected from the circuit?

I mean if R3 increased to 1Meg Ohm, this I called it as no load.

 

My target is to maintain the output voltage constant at both with load and without load.

hi pp,
Why is it an operational requirement to obtain a zero change between a loaded and unloaded voltage output????

E

 

The LM7805 has a max drop of 50mV going from 250mA to 750mA. Why do you expect your simple circuit to do better than that?

 

The LM7805 has a max drop of 50mV going from 250mV to 750mV.

Typo? did you man 250-750mA

 

if the R3 is load... what do you mean exactly by "without load"? R3 removed from circuit?
don't leave inputs floating. here is an example of that (added R2 to be permanent "minimum load")


also OpAmp is good at things like high gain but terrible at driving load. absolute maximum is 10mA, you should really not expect more than 5mA from it. and 5mA base current may not cut it... you better make that Q1 a Darlington.