I have designed a transistor matcher which is basically just an inverting transistor stage.
The DUT base is driven from an TL084 through a 1 k resistor.
I chose the 1k so as not to load the op amp output.
The DUT npn base is driven fine through the 1K resistor with a nice clean collector waveform.
However, the DUT pnp gets quite a bit of oscillation when pnp transistor gets turned on. The oscillation is at the op amp output too which is strange.
The only difference is npn is driven from +ve supply and pnp driven from -ve supply.
I can tune out the oscillation by replacing 1k with 3k3 but anything less than 3k3 results in oscillation.
The DUT base is driven from an TL084 through a 1 k resistor.
I chose the 1k so as not to load the op amp output.
The DUT npn base is driven fine through the 1K resistor with a nice clean collector waveform.
However, the DUT pnp gets quite a bit of oscillation when pnp transistor gets turned on. The oscillation is at the op amp output too which is strange.
The only difference is npn is driven from +ve supply and pnp driven from -ve supply.
I can tune out the oscillation by replacing 1k with 3k3 but anything less than 3k3 results in oscillation.
On your schematic, split resistor R23 into two series resistors of equal value, and connect a 330nF capacitor from the midpoint to the collector of Q24. Voila.
Hmmm....the TL084 is a strange choice...if you'll look at the datasheet, it really starts suffering at driving anything below about 10K ohms...
You don't suppose someone would ask a circuit question without showing the schematic, would they?
Well that explains a lot 🙂
Maybe the thread should just be closed seeing as it is all meaningless.
Maybe the thread should just be closed seeing as it is all meaningless.
Jan I think that Mark just made it up on the spot as a joke because there was no schematic 😉
Tony.
Tony.
Had another what I thought was op amp problem today.
I have a 2 channel USB scope and the 2 channels wouldnt zero to each other.
So cursed the op amp circuit again as I thought resistor tolerances into op amp was causing the problem. But no output of both op amps is minimal.
Turns out it is inaccuracies in dual A2D converter.
So messed up the pcb, I need 2 pots now to zero out the channels.
Also output wasnt going to zero on screen.
I have a diode in the feedback loop of op amp to prevent it going below zero.
Wasnt accurate enough and I really just need a current limiting resistor on the output.
The joys of design work. Another day and more learned.
I have a 2 channel USB scope and the 2 channels wouldnt zero to each other.
So cursed the op amp circuit again as I thought resistor tolerances into op amp was causing the problem. But no output of both op amps is minimal.
Turns out it is inaccuracies in dual A2D converter.
So messed up the pcb, I need 2 pots now to zero out the channels.
Also output wasnt going to zero on screen.
I have a diode in the feedback loop of op amp to prevent it going below zero.
Wasnt accurate enough and I really just need a current limiting resistor on the output.
The joys of design work. Another day and more learned.