Current mirror transistor saturation

current mirror transistor

Hi!


I have some problems with amplifier that I made, burn out, then repair
Here is it

**broken link removed**

This is not part of original schematic, but can explain situation. Left transistor is in saturation, emitor-base voltage is about 0.65-0.7V, but emitor-collector voltage is nearly 0V. What can cause this problem here?!


Thanks! I hope you can help me.

p.s. sorry for my bad English

transistor saturation

Current mirrors require that the transistors be well matched and that they are thermally tied together.

Since you stated that you repaired the circuit, I can only guess that these are individual transistors, not an IC chip. I would guess that the betas are not well matched and/or the transistors are not thermally bonded to maintain the same temperature.

current mirror transistors

Yes, transistors are individual, as in many other amplifiers, 2xBC6xx.
I change it both at once, and this work well before the first problem (non-grounded amp plugged in the TV out), but after changing all transistors this problem remain.

current mirror circuit schematic

BJT transistors are current amplifiers. For a given base current, the transistor will adjust its C-E ON resistance until the beta equation is satisfied.

Ok, since C-E is too low, 0V, there are two explanations. First, perhaps the base current is too high which is driving the transistor into saturation. Replacing the transistors on the non-inverting half should fix that.
Second possibility is that the load on the inverting side cannot pass enough current and thus pushing C-E to zero. Replacing transistors on the inverting half should fix that.

If you have replaced all the transistors, then you have to first double check that all the NPN and PNP are installed in the correct order and nothing is reversed. Next inspect all the solder for bridges or an unsoldered transistor leg. Finally, start checking the discrete resistors and diodes. I believe your posted schematic is just an example and not the real circuit. Look for small value resistors in series with the emitters of the BJTs. These are often used for balancing and during failures will either burn open or change value. Small signal bias diodes often also go open or leaky.

Does the circuit have any balance or adjustment pots? If so these variable resistors may need to be reset to adjust for the new transistors.

calculate transistor saturation

In a short, Vce = 0 has two possible reasons:
1. The respective transistor is shorted, can be easily checked with a meter
2. No current is flowing into it from the differential pair. Either because of a defective transistor in that place or diff. amp input voltage simply shuts it off.

differential pair tubes sbalanced

banjo said:

If you have replaced all the transistors, then you have to first double check that all the NPN and PNP are installed in the correct order and nothing is reversed. Next inspect all the solder for bridges or an unsoldered transistor leg. Finally, start checking the discrete resistors and diodes. I believe your posted schematic is just an example and not the real circuit. Look for small value resistors in series with the emitters of the BJTs. These are often used for balancing and during failures will either burn open or change value. Small signal bias diodes often also go open or leaky.

Does the circuit have any balance or adjustment pots? If so these variable resistors may need to be reset to adjust for the new transistors.

Click to expand...

Yes, this schematic is just an example. Real circuit has tubes instead transistors in differential pair and 470R resistor in emitors of transistors in current mirror, it's not directly connected with +V. Resistors and PCB are OK, I check that carefully.

FvM said:

In a short, Vce = 0 has two possible reasons:
1. The respective transistor is shorted, can be easily checked with a meter
2. No current is flowing into it from the differential pair. Either because of a defective transistor in that place or diff. amp input voltage simply shuts it off.

Click to expand...

1. I checked, it's not shorted
2. A equal current is flowing in both sides of diff pair, about 5mA each.



Thanks!

current mirror in saturation

Are you sure the vacuum tubes in the differential pair are OK? It would seem like a weak tube in the inverting side would cause this condition.

If the C-E is very low and the transistor is not shorted, it means that the transistor cannot pass all the current that it needs to in order to satisfy the beta equation. Since the currents are roughly equal in each side, it would seem that the inverting diff amp is too weak and provides too little load on the BJT.

Are the grids of the tubes AC coupled? Perhaps you have a leaky capacitor that is upsetting the grid bias.

What are the grid to cathode voltages on each side?

Since this is an audio amp, can you feed in a sinewave to either input and look at the output? This may show clipping that would point to the problem.

tube amp current mirror

Inv. side catode: ~2.2V
Non-inv. side catode: ~2.3V
Grids: 0V


I don't have proper equipment to test tubes and amp with sinewave, only basic instruments.

mirror transistor

With only basic instruments, you have to start making experiments. You know the expected plate voltage of the tube and you know the current through the tube. By ohms law, (V/I), you can calculate the equilvalent resistance. Now pull the tube in the inverting side and replace it with a resistor of approximately the value calculated. Connect the resistor from plate to cathode. The other tube elements you can leave open. If the tube has series heaters, or the package is a dual tube, then you may have to just disconnect the plate and cathod instead. Power it up and measure the voltage at the C-E voltage. If it is still 0V, then the problem seems to be in the current mirror. If the voltage returns to something close to the expected value, the problem seems to be in the lower differential pair.

current mirrors audio

After replacing the tube (I have two tubes) transistor is not in saturation anymore
Thanks!!!