Transistor

[hupsenfg]

Hi can someone teach me how to solve this question

 

[ericgibbs]

hi hup.
Let me ask you a question:
In the circuit with the 5k collector resistor, what is the collector current.?
Eric

 

[hupsenfg]

It is same as the emitter current which is (1.7-0.7)/1k= 0.001A

 

[ericgibbs]

hi,
OK, so it is 1 milli-Amp
So assume that Vbb and Ic remains the same and now we change the 5K to 11K, what will the Collector voltage be now.?
E

 

[hupsenfg]

Collector Voltage will be 12v- (11K*0.001A)= 1V

 

[ericgibbs]

OK,
So Vc=1v and Ve=1v, Vbb =1.7v
Assuming a transistor with a Vbe On of +0.7v.
With those values and the negative feedback action of the 1K in the Emitter, the transistor will be just out of saturation.
Do you follow that OK.?
You should now be able to prove that mathematically.
E

 

[hupsenfg]

I am just unsure about the 2nd part where it mentions the voltage to get the maximum dynamic range.

I get it is in the middle between saturation and cut-off, But I do not know how to do get there. (i.e. I don't know how to get the answer 1.2v)

 

[ericgibbs]

hi,
You know that Collector voltage could ideally swing from 12V down to 1V [ the Vsat+Ve] so half way between 12v and 1v is 5.5v.

Look thru this tutorial link and come back with any queries, it explains the operation
Example #1.
http://www.electronics-tutorials.ws/amplifier/emitter-resistance.html

E

 

[ericgibbs]

I get it is in the middle between saturation and cut-off, But I do not know how to do get there. (i.e. I don't know how to get the answer 1.2v)

hi,
The middle of Vcut off and Vsat is Vsupply minus Vsat which is 12v -1v = 11v/2 = 5.5V

So you have to calculate the value of Collector current that will give a voltage drop across the 11k of 5.5V.
Assume the same current thru the 1k Emitter resistor, from that current you can calculate the Ve and adding a transistor Vbe 0.7v to Ve will give Vbb voltage required.

The Gain of the Common Emitter is Collector Resistor divided by the Emitter resistor, 11k/1k =11

E

 

[hupsenfg]

The question asked for the change the of voltage value of Vbb though. So there is no change in 12 V i am assuming?

That being said, thank you for the link. About the link, IF VBE decreases from an increased voltage source, how does that affect VC?

 

[ericgibbs]

hi
The change in Vbb is to reduce the collector current so that the collector voltage is set to 12v -1v= 11v/2, it is now 0.5mA.

This means the Ve voltage is now only 0.5v, so add 0.7v gives 1.2V for Vbb.

E

 

[ericgibbs]

hi hup,
This is an LTSpice simulation of the circuit.
Note: I have used a value of 0.6v for Vb not 0.7v, this is more accurate, the Vc swings close to 12v and 1v, ie: +/-5.5v
E

 

[Ratchit]

I am just unsure about the 2nd part where it mentions the voltage to get the maximum dynamic range.

I get it is in the middle between saturation and cut-off, But I do not know how to do get there. (i.e. I don't know how to get the answer 1.2v)

First, find the saturation current. Solve 12 - 11000*I = 1000*I which gives I = 1 ma for saturation current. One half of 1 ma is where the transistor should be for max dynamic range. Ve = 1000*0.5 ma = 0.5 volts. Add 0.7 gives 1.2 volts.

Ratch