CCS acting strange. What's wrong?

ventle · 21st November 2009, 03:49 PM

The constant current source in the attached schematics is part of my quite common lin three-stage class B amplifier design. It's supposed to feed the input stage (Load 1, LTP), and the VAS stage (Load 2, darlington paired 2SD669). There's also a bootstrap capacitor (47uF) connected between the +70V rail and the junction between resistors R4 and R5, I forgot to draw this in the schematics.

The problem is that the R1 resistor, which is supposed to set the quiescent current of the VAS at approx 6.7 mA (0,67V/100R). As I've drawn into the schematics, I measured the voltage across R1 to be only 134mV.

The next problem is that the transistor Q1 is turned hard on making it drop only 6mV across its collector and emitter (Vce). This leads to the high-side ouput device (2SK1058) being turned hard on as well, while the low-side device (2SJ162) is hard off. I measure a DC voltage of 20V at the output of the amplifier...

I didn't draw the entire schematics because I believe the error can be found somewhere within the CCS drawn here. Anyone see any obvious design flaws? I attempted to remove R2 and R6 because some designs have and some don't have these resistors in place, but it didn't make any difference.

Lumba Ogir · 21st November 2009, 04:15 PM

ventle,
you need two separate constant current sources, like this:

Elvee · 21st November 2009, 04:26 PM

I'd say your VAS is not sinking a sufficient current to keep the CCS in linear mode. Maybe no current at all. Perhaps a latch-up issue caused by the entanglement of the two sources.

Lumba Ogir · 21st November 2009, 04:39 PM

Elvee,

Quote:

I'd say your VAS is not sinking a sufficient current to keep the CCS in linear mode. Maybe no current at all. Perhaps a latch-up issue caused by the entanglement of the two sources.

No. The circuit in post #1 simply cannot work as intended.

ventle · 21st November 2009, 04:45 PM

Thanks Lumba.

What is the purpose of the 10k resistor (R2) in your schematics? Is it for linearizing, limiting current, or just simulating a load? In the latter case, should I omit it?

Lumba Ogir · 21st November 2009, 04:58 PM

ventle,
R2 (10k) lowers the power dissipation in T1 so you can use a better device. R2 in your circuit is not needed.

Elvee · 21st November 2009, 05:46 PM

Quote:

Originally Posted by Lumba Ogir

Elvee,

No. The circuit in post #1 simply cannot work as intended.

Could you elaborate?

Lumba Ogir · 21st November 2009, 06:03 PM

Elvee,
I personally would stick to two separate constant current sources of this type (ring of two), to provide a stable and low Vref for R7 and R1 (in post #1).

Lumba Ogir · 21st November 2009, 06:22 PM

Elvee,
...instead of this:

Elvee · 21st November 2009, 06:22 PM

This circuit is not necessarily my first choice either, but it should work more or less as intended.
When it works properly, it will be time to think of something better.

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21st November 2009, 04:26 PM	#3
Elvee diyAudio Member Join Date: Sep 2006	I'd say your VAS is not sinking a sufficient current to keep the CCS in linear mode. Maybe no current at all. Perhaps a latch-up issue caused by the entanglement of the two sources.

21st November 2009, 04:45 PM	#5
ventle diyAudio Member Join Date: Jan 2008	Thanks Lumba. What is the purpose of the 10k resistor (R2) in your schematics? Is it for linearizing, limiting current, or just simulating a load? In the latter case, should I omit it?

21st November 2009, 04:58 PM	#6
Lumba Ogir Banned Join Date: Apr 2005 Location: Stockholm	ventle, R2 (10k) lowers the power dissipation in T1 so you can use a better device. R2 in your circuit is not needed.

21st November 2009, 06:03 PM	#8
Lumba Ogir Banned Join Date: Apr 2005 Location: Stockholm	Elvee, I personally would stick to two separate constant current sources of this type (ring of two), to provide a stable and low Vref for R7 and R1 (in post #1).

21st November 2009, 06:22 PM	#10
Elvee diyAudio Member Join Date: Sep 2006	This circuit is not necessarily my first choice either, but it should work more or less as intended. When it works properly, it will be time to think of something better.