Electrical – Why is the collector- emitter voltage (Vce) not getting biased


This question is an extension for my previously asked question here:
Why is the voltage across collector and emitter pushing itself to 0.7 volts?

A colleague of mine suggested that I take a look at the stability factor for the circuit in that question. I calculated it and found it to be 284, which is very high and bad, and I thought that this is what was driving the transistor into saturation. But then re-did my calculations by assuming a lower stability factor of 10. From this, I got new values of resistors as follows:

R1 = 4.7 K ohms

R2 = 1.2 K ohms

Rc = 2.2 K ohms

Re = 100 ohms

After making these changes, when I tested the DC conditions by applying a supply voltage of 5 volts, there were no changes in the results. The Vce voltage still remains at 0.7 volts, i.e almost in saturation, contrast to my expected value of 2.5 volts. Here are the various voltage drops I get:

Across R1 : 4.6 volts

Across R2: 0.4 volts

Across Rc: 4.2 volts

Across Re: 0.2 volts

I have even tried changing the transistor. The transistor I used is 2N3904C NPN.
Please enlighten me and provide me with a solution on how to overcome this.

Best Answer

On this question, and on your previous question, you keep getting 0.7 volts Vce.

Perhaps you have swapped the base and collector leads. This would put the base emitter leads across your "emitter" resistor, resulting in 0.7 volts no matter how you bias the resistors R1 and R2.


enter image description here

EDIT : If you have connected transistor properly, you can place a jumper across your R2 (short it out, making it zero ohms), and Vce should rise dramatically. Do not leave the jumper installed after this test.

After satisfying that your circuit is wired properly, increase value of R1 (perhaps to 6.8K) and increase in small steps until you see Vce begin to rise in voltage.

I have added a schematic to represent what the OP has in his previous question, and with his current values.


simulate this circuit – Schematic created using CircuitLab