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lumanauw 18th February 2005 03:55 AM

What happen with this transistor?
 
2 Attachment(s)
I made an experiment with CCS. The supply is +/-15V, and the reference for the CCS is 2xIN4148 drop=1V3.

In drawing I, all works fine, the A CCS gives 3.2mA to 1k load, and CCS B gives 6.95mA to 1k load. The current, voltages are measured, and nothing is unusual.

In drawing II, something strange is happening. Here I connected both CCS's with 1k load. The A CCS (the smaller one) still working fine, but CCS B doesn't work normally.

In the CCS B (of drawing II), the drop of the IN4148 only 1V2(from 1V3).
The current passing through 1k load is 3.2mA (exactly the same as the upper/smaller) CCS value. But the drop in 100ohm resistor of B CCS is 0V53, that means there is 5.3mA current here.

Also, the VBE of CCS B increasing to 0V67 (from 0V58).
If in CCS B, in the transistor, the collector has 3.2mA passing thru, but in the emitor there is 5.3mA passing thru, where is 2.1mA (5.3-3.2) coming from? Is it coming from base, that makes the drop of 2xIN4148 only 1V2?

A transistor can do this? Passing big current from base to emitors but not destroying the transistor itself?

1. The VBE figure seems doesn't relate with current passing from C to E. The current passing in VBE=0V67 can be less than when VBE is only 0V58

2. Or this happens because there is big current (2.1mA) passing from base to emitor junction?

Leolabs 18th February 2005 08:47 AM

YO!It looks like an electronic engineering homework to me!

Leolabs 18th February 2005 08:49 AM

Anyway please like me know the part number of the NPN and PNP;)

jpnascim 18th February 2005 09:49 AM

lumanauw,

I am a novice in electronics but I'll try (if I am wrong I can still learn !)

It looks like transistor B in circuit II is saturated. DC current gain is too low. The high base current explains the higher Vbe voltage as this B-E junction has a small resistance and therefore, Vbe drop increases with current too (just a little). Also, another consequence of a high base current is the decreased voltage drop in diodes).

Try to measure Vce of this transistor. If it is close to 0 then it is saturated.

Can you tell us which transistor are you using ?

Best regards,

Joćo Pedro

moamps 18th February 2005 09:50 AM

Re: What happen with this transistor?
 
Quote:

Originally posted by lumanauw
In drawing II, something strange is happening. Here I connected both CCS's with 1k load. The A CCS (the smaller one) still working fine, but CCS B doesn't work normally.
Hi,

Set current in both CCS on same value. CCS B transistor doesn't work in active operation area in your setup.

Regards,
Milan

Upupa Epops 18th February 2005 10:07 AM

If you connect two current sources with different current in series, cross current will be given by current of lower current source.

lumanauw 18th February 2005 03:22 PM

Quote:

It looks like transistor B in circuit II is saturated
What is "transistor saturating"? Is it very bad for audio reproduction? Is it the same when people talks about clipping?
Quote:

DC current gain is too low
What makes DC current gain?

What condition makes the base draws so much current, but do not imply in collector current?

The transistors are BD139 and BD140.

Quote:

Set current in both CCS on same value. CCS B transistor doesn't work in active operation area in your setup.
Actually, I put these configuration on a purpose. I wanted to know what happens in CCS B.

Quote:

If you connect two current sources with different current in series, cross current will be given by current of lower current source.
Yes, It's very clear in the experiment result. But why is the lower CCS transistor not acting normally like ordinary transistor (HFE definition)? At first I really hope the lower CCS doesn't act like this.

I'm searching for a simple cct (transistor, R or dioda) that acts like a "minimal voltage limiter". That means if an input is injected to this simple cct, the voltage can rise, but if there is no input, or the input is negative (sucking) signal, the voltage can hold at a certain voltage (not dropping to 0 or turning negative). It has tobe adjustable too, not fixed like zener.

MikeB 18th February 2005 03:58 PM

Hi lumanauw !

This looks like a typical situation of reversebiasing.
If a transistor does not "get rid" of the needed current
through the collector, it does through base.
Or in other words, you try to get a negative vce, which
is not possible. That's the saturation.
Unless you don't exceed the maxrating for basecurrent
given in the datasheet, the transistor does not get
destroyed.

In the case of reversebiasing a transistor does not
really work. This typically happens if an amplifier clips.

Mike

X.G. 18th February 2005 04:25 PM

Re: What happen with this transistor?
 
Quote:

Originally posted by Upupa Epops
If you connect two current sources with different current in series, cross current will be given by current of lower current source.
I follow Mr. Upupa Epops;)

Quote:

Originally posted by lumanauw
1. The VBE figure seems doesn't relate with current passing from C to E. The current passing in VBE=0V67 can be less than when VBE is only 0V58

No.

Don't forget the Ic of BJT is relate with the Vce...you should check the Vce,but maybe can not get the correct value of Vce in practice.

cheers.

X.G.

X.G. 18th February 2005 05:02 PM

Re: Re: What happen with this transistor?
 
Quote:

Originally posted by X.G.

Don't forget the Ic of BJT is relate with the Vce...you should check the Vce,but maybe can not get the correct value of Vce in practice.

cheers.

X.G.

sorry,I take a mistake:bigeyes:

I should say:the NPN tr works in saturation area,and the PNP works in line area,not like the left circuit which both of trs work in line area.their Vce is clearly different


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