# Common Base

Discussion in 'Electronics Homework Help' started by Wade Yaden, Jun 25, 2016.

7
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Jun 25, 2016
1. For the circuit shown in Figure 3 below, draw the DC load line and locate its quiescent or DC working point. Show all work.
I have never done a common base problem before and our book does not go over this. Our book says to mentally "short" out the transistor to find Current Saturation Point. And "open" transistor to find the Voltage Cutoff. Once I short the transistor not sure how to calculate the current. Little lost on this..Just want to be able to understand what to do...

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
if you short out the transistor, what is the combined voltage across Vb and Vl?

for the purposes of calculation you might also want to include Ib in your final calculation, but this may be both unknown and small

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Jun 25, 2016
Thats the question Im asking. If I short the transistor out. How do I treat each resistor (label). The same - different. This is our first week in transistors. Can I do KVL in a loop and treat them the same. All the problems I have to reference in the one chaper we started are all common emitter.

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
Take the transistor out and place a line between where the emitter and collector were connected.

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Jun 25, 2016
And?? I think we have established the part of shorting out the transistor. I stated this in the original post. Yes a line across the collector and emitter. How do I treat the problem( 2 voltages 2 resistors). If that was misunderstood. That was my original question.

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
Two voltages mean one voltage difference does it not?

7. ### Herschel Peeler

401
66
Feb 21, 2016
If you take the transistor out you lose the E-B voltage. So short Emitter to Collector. W
Is the circuit drawn correctly? I would expect the higher resistor value to be on the Base. NPN or PNP transistor?

I would expect a circuit more like this.

Last edited: Jun 25, 2016
8. ### Laplace

1,252
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Apr 4, 2010

But in reviewing the circuit, does it not seem a little strange that VEE is driving the collector and Vcc is driving the emitter?

(*steve*) likes this.

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Jun 25, 2016
Picture was copied straight from practice problem..

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Jun 25, 2016
Sheet says NPN

7
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Jun 25, 2016
Even tried to play around with it in Multisim and I wasnt able to pick up any readings anywhere.

12. ### Herschel Peeler

401
66
Feb 21, 2016
You have an NPN transistor with the emitter more positive than the base. It will never turn on. Bad picture.

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Jun 25, 2016
I agree..it's a bad pic...Yes that was what I was getting in Multisim (0) ...when I flip the transistor (which is the way I think it should be drawn) I start getting data. Didn't catch that Vcc and Vee label...waiting from teacher on response...

14. ### Herschel Peeler

401
66
Feb 21, 2016
Okay, having flipped it, go back to your questions. What will the voltage on the emitter be? Assuming the output is on the collector (it doesn't show that either). Shorting the transistor (transistor on and in saturation) what will the voltage on the collector be? What is the maximum collector current? With the transistor fully off (ground the emitter) what will the voltage on the collector be? What will the minimum collector current be?
Given the values shown what would the emitter-base current be? What might the collector current be? What is the current gain of the 2N1711 at that collector current?

This is just a badly made question if you ask me. The transistor is flipped. No output is shown. I would have made the emitter resistor about 430,000 ohms.

Interesting questions ...
What is the emitter-base current?
what is the maximum collector current?
What is the gain of the 2N1711 at that collector current?
At that gain and that emitter-base current what might the collector current be?

Last edited: Jun 27, 2016
15. ### Herschel Peeler

401
66
Feb 21, 2016
Not that it is really relevant to your question,, just because it is a nice place to mention transistor characteristics ... The data sheet for a 2N1711 gives the following info on the gain of the transistor.

hFE = 20 (@ IC = 10 µA)
hFE = 35 (@ IC = 0.10 mA)
hFE = 75 (@ IC = 10 mA)
hFE = 100 to 300 (@ IC = 150 mA)
hFE = 40 (@ IC = 500 mA)

The entry at 150 mA shows how much variation there could be for any given transistor. At low collector currents the gain is small. It peaks at some point, then goes down again at saturation. Given any specific current we can only interpolate what it might be at any specific collector current. So, hFE about 50 (@ IC = 6.66 mA maybe)

VCEsat = 500 mV (IC – 150 mA, IB = 15 mA) We are well below a collector current of 150 mA so we can at least realize VCE will not go all the way to zero volts. We inject a small error when we short the transistor Emitter to collector for setting the load line.

VBEsat = 1.3 V (IC – 150 mA, IB = 15 mA) We are well below 15 mA on IB but we can estimate our VBE to be about 600 mV.

If you actually have a 2N1711 you can confirm these statistics.