Hi
can some one please advise on the best simulation of this circuit ,not through the scope but by the use of one of the analyses methods on multisim , i have experimented but can not seem to get anywhere near the ideal output ,am i doing something wrong or have i not been using the correct settings ? just need a helping hand or some sound advice on how to proceed.

cheers
spark 8217

 

The -12V on the base cuts the transistor off. Why do you have it biased like that?

 

Hi
Thanks for the reply this is a task set and the circuit was given to simulate build and write a report about that circuit and other various circuits, is that why the output is not like a typical tuned amp circuit ? i spoke to my tutors and they admitted it was a poor circuit and then when i simulated it i was not quite sure on the output , hence my question here for clarification.


cheers
spark8217

 

Hi
Thanks for the reply this is a task set and the circuit was given to simulate build and write a report about that circuit and other various circuits, is that why the output is not like a typical tuned amp circuit ? i spoke to my tutors and they admitted it was a poor circuit and then when i simulated it i was not quite sure on the output , hence my question here for clarification.


cheers
spark8217

This is apparently homework.
Are you supposed to change the design so that it works? If not, what further is expected of you?

 

Cheers
Ron
apparently not, the only tasks where as follows
1- describe operation
2-sketch & explain frequency response
3-produce gain /freq response
4- determine gain bandwith
5 produce graph of gain versus / freq

for my own satisfaction would i presume that by putting another resistor with r1 and providing a voltage divider at the base of the transistor would improve the circuit ? and bring the cap c4 across R2 and increase its value ?.

i have calculated BW=297.62
Q=168
GB=60661207.18
AV=-64171.12299
ZD=534759 Ohm
20log(64171)=96.14677614 db

cheers
spark8217

 

Cheers
Ron
apparently not, the only tasks where as follows
1- describe operation
2-sketch & explain frequency response
3-produce gain /freq response
4- determine gain bandwith
5 produce graph of gain versus / freq

for my own satisfaction would i presume that by putting another resistor with r1 and providing a voltage divider at the base of the transistor would improve the circuit ? and bring the cap c4 across R2 and increase its value ?.

i have calculated BW=297.62
Q=168
GB=60661207.18
AV=-64171.12299
ZD=534759 Ohm
20log(64171)=96.14677614 db

cheers
spark8217

How did you get those answers? The gain is basically zero, so the other stuff is irrelevant.
If you rebias the amplifier so that it has gain, you have to include the loading of R3, which affects Q and gain.
To rebias, you don't need the -12V supply. You just need a voltage divider to the base from +12V and ground.

 

so basiclly my only option is to re -design the circuit so it has bias to get gain ? i got the answers after trying to calculate the parameters of the circuit because i was not happy with the response or the output when i simulated it , and asked the question also and was said it was not a good circuit , i showed my working out to the tutor which he said i made a couple of errors and he helped me calculate to the parameters i stated above of which i agree are irelivante if the gain is zero , we had measured previously the gain of 0.9 ish which backs up your suggestions and the circuit was behaving odd to say the least.
cheers
gringo 8217

 

So your tutor helped you get those answers? Where are you going to school?
Are you sure the schematic is drawn correctly?
You cannot get a gain of 0.9 with that circuit. If you removed the 10k load, you could get a little"gain", but it would not be due to transistor action. It would be a result of coupling through the collector-base capacitance.

If it is drawn correctly, then I have a question:
Have you heard the expression, "The blind leading the blind"?

 

hi
Ron
Yes indeed i have !! and the thought had crossed my mind ,but that doesnt solve my problem and makes it even harder asking the question again, i knew something was not quite right when the transistor was removed and the circuit was behaving oddly ,and not been able to get any output at all near a ideal small tune amplifier waveform im now at a loss.

cheers
spark 8217

 

The whole thing smells bad. I do not buy "poor circuit".

Even a minimum of idea about all this would preclude throwing those -12V, even if the idea was a random circuit. That much of randomness is pure nonsense. What translates to waste of time of the worst class where nothing is learnt.

Reduction to the absurd pertains to theorems only.

Are you sure it was the instructor...?

 

hi
Ron
Yes indeed i have !! and the thought had crossed my mind ,but that doesnt solve my problem and makes it even harder asking the question again, i knew something was not quite right when the transistor was removed and the circuit was behaving oddly ,and not been able to get any output at all near a ideal small tune amplifier waveform im now at a loss.

cheers
spark 8217

Have you been taught how to design a class A common emitter amplifier, with voltage-divider base biasing?
Do you know how to calculate the gain and the output impedance of the resulting amplifier?

 

100% positive ,i have checked the circuit and double checked it ,it is as drawn on my schematic as it is on the assignment sheet along with the questions. i can attach if required.

cheers
spark 8217

 

Did you see post #11?

 

hi
Ron
yes i do know how to calculate them

 

hi
Ron
yes i do know how to calculate them

OK, then you're all set!