Troubleshooting problems.

[paulmdrdo]

Hello! I'm having a problem answering the following objective questions from my book about troubleshooting bjt amplifiers.



For item 19, I answered A and D
Assuming the coupling capacitor is open (either the input coupling cap or output coupling cap) the ac output voltage would be zero. But if the bypass capacitor is open the ac output voltage just decreases but not go to zero. But the answer in my book is A. This is confusing because the question says "any capacitor is open", there are two possible answer A and D.


For item 23, I answered C.
My reason is, if the emitter resistor is open and by using a.c equivalent of the circuit (where we short the bypass capacitor to ground) the open emitter resistor won't matter. And the resistance looking in at the base is Br'e(this is the same resistance if the emitter resistor i not open). Therefore the ac input voltage remains the same. But in my book the answer is B

For item 24, I answered C
Here, since Rc is in the output circuit it does not affect the ac input voltage. But in the book the answer is A.

For item 25, I answered c.
In a.c equivalent circuit tge emitter is already shorted to ground. Again, the resistance looking in at the base s Br'e( this is the same when the bypass cap is not shorted) therefore C was my answer. The only way(I know) ac input voltage would decrease is when the input resistance looking in at the base decreases.


For item 27, I answered B.
But not quite sure if my analysis is correct.

Please share to me your thoughts on this and any correction in my analysis would be appreciated.

 

[Audioguru]

It looks like the answers in the book are WRONG.

 

[KlausST]

Hi,

Item 19:
The question is: Where do you measure the output voltage. If it is at Rl, then the output voltage is zero.

Item 23:
With the emitter resistor the base current is not zero. But when the emitter is open, then i expect no base current, too.
Without current there will be no voltage drop in the base driving circuit (source, resistor, capacitor). I'd agree the base AC voltage increases.

Item24:
With the collector connected properly, the current in the emitter is the base current plus the collector current. This causes a voltage drop (AC) in the emitter resistor.
Now without the collector current there is only base current through the emitter resistor. The current is much lower than before, and therefore the voltage at the emitter is lower than before. Because the ACbase voltage is about the same as the AC emitter voltage...I again agree, that the AC basevoltage will decrease.

(...low batteries, sorry)

Klaus

 

[CataM]

For item 24, I answered C
Here, since Rc is in the output circuit it does not affect the ac input voltage. But in the book the answer is A.

Item24:
With the collector connected properly, the current in the emitter is the base current plus the collector current. This causes a voltage drop (AC) in the emitter resistor.
Now without the collector current there is only base current through the emitter resistor. The current is much lower than before, and therefore the voltage at the emitter is lower than before. Because the ACbase voltage is about the same as the AC emitter voltage...I again agree, that the AC basevoltage will decrease.

I will refer to ITEM 24 and Medium Frequencies analysis (all capacitors shorted):

In AC analysis, for circuit a) anwser = C. Meanwhile for circuit b) answer=B because the base of the first transistor remains the same but for the second transistor increases.

PS:I have considered output in the RL resistor.

 

[paulmdrdo]

Hi,

Item 19:
The question is: Where do you measure the output voltage. If it is at Rl, then the output voltage is zero.

Item 23:
With the emitter resistor the base current is not zero. But when the emitter is open, then i expect no base current, too.
Without current there will be no voltage drop in the base driving circuit (source, resistor, capacitor). I'd agree the base AC voltage increases.

Item24:
With the collector connected properly, the current in the emitter is the base current plus the collector current. This causes a voltage drop (AC) in the emitter resistor.
Now without the collector current there is only base current through the emitter resistor. The current is much lower than before, and therefore the voltage at the emitter is lower than before. Because the ACbase voltage is about the same as the AC emitter voltage...I again agree, that the AC basevoltage will decrease.

(...low batteries, sorry)

Klaus

How come there's an ac voltage in emitter in 24? I thought when we consider the ac equivalent circuit the emitter reistor is bypassed hence the ac current would go directly to ground thus there would be no ac voltage?

 

[KlausST]

Hi,

How come there's an ac voltage in emitter in 24? I thought when we consider the ac equivalent circuit the emitter reistor is bypassed hence the ac current would go directly to ground thus there would be no ac voltage?

There is a (non zero) impedance from emitter to Gnd. So every current generates a voltage. Maybe it is small..

 

[CataM]

There is a (non zero) impedance from emitter to Gnd. So every current generates a voltage. Maybe it is small..

Yes, if considering the caps. But here is not supposed to work in the Mid band ?

 

[Audioguru]

At a mid frequency, a bypassed emitter has no signal.