2sa970/2sc2240 sudden HFE DIP

[haseeb]

Hi All,

I was working with an input stage using CFP method .

I found there is sudden drop in hfe, the gain is very minimal.

Then tried to touch some pins of the active and passive devices for any dry solder and for some pin there was sudden increase in gain.

I checked for dry solder which is just fine.

Then I tried to drill down what is causing the dip in gain found that when I touch collector pin or emmiter pin of any 2sa970 or 2sc2240 there was a dip in gain and it never come normal until retouch or short circuit Emiiter-collector pin.

Why is this happenning any Idea.

I am working on 34+ve 34-ve rails

regards,
Haseeb

[unclejed613]

it might help to see a schematic of the circuit in question..... transistors don't just change gain for no apparent reason unless they are being over or under biased.

[nigelwright7557]

Originally posted by haseeb
Hi All,

I was working with an input stage using CFP method .

I found there is sudden drop in hfe, the gain is very minimal.

Then tried to touch some pins of the active and passive devices for any dry solder and for some pin there was sudden increase in gain.

I checked for dry solder which is just fine.

Then I tried to drill down what is causing the dip in gain found that when I touch collector pin or emmiter pin of any 2sa970 or 2sc2240 there was a dip in gain and it never come normal until retouch or short circuit Emiiter-collector pin.

Why is this happenning any Idea.

I am working on 34+ve 34-ve rails

regards,
Haseeb

I had a similar problem on one of my amp builds.
The output would start at zero then slowly go to B- !
One of the input resistors had a dry joint.

[Elvee]

Originally posted by haseeb
Hi All,

I was working with an input stage using CFP method .

I found there is sudden drop in hfe, the gain is very minimal.

Then tried to touch some pins of the active and passive devices for any dry solder and for some pin there was sudden increase in gain.

I checked for dry solder which is just fine.

Then I tried to drill down what is causing the dip in gain found that when I touch collector pin or emmiter pin of any 2sa970 or 2sc2240 there was a dip in gain and it never come normal until retouch or short circuit Emiiter-collector pin.

Why is this happenning any Idea.

I am working on 34+ve 34-ve rails

regards,
Haseeb

Looks like you are experiencing VHF or UHF oscillations: this is typical behaviour when a circuit is on the verge of oscillation. When the circuit oscillates, there is an apparent drop in DC/LF gain.
To ascertain this is the case, you can solder 1nF ceramic caps directly (short leads or no leads) across the base and emitter of each transistor.
This will quash any spurious oscillation and have little impact on the LF behaviour.
LV

[haseeb]

Hi Elvee,

Looks you are exactly there.

Yes, there is LF gain.

Yes , I too felt the same and tried to put some pf valued cap across collecter and emmiter , I found there is change in frequency.

I'll try what you suggested and get back soon.

Thanks Elvee

[haseeb]

Hi Elvee,

I tried your work around but did not helped.

I am redesigning my circuit.

Regards,
Haseeb