Voltage amplifier transistor circuit (diagram provided)

[scilearner]

Hello guys,

 

I'm struggling with these type of circuits. Here is my question. I have drawn a diagram in paint

 

 

When the input AC voltage= 0 v the collector current is found to be 1.0 mA. The current gain of the amplifier is 200. There must be 0.7 V between B and E before current can flow from the collector to the emmiter. What is the resistance of Rb

 

Well I know that Ai= Ic\Ib and I can find Ib but I don't know how to use this information. My understanding of series and parallel circuits is good but I don't know how this type of circuit works. If anyone can give me a detailed step by step of how this system works I'll be reallly really greatful

[YT2095]

treat Ra and Rb as if you were setting up a simple PD.

use an Ideal value for the voltage, say 1 volt for instance, then work out the resister values that will give you .7v across Ra.

 

then multiply that by 9 (since you`r using 9 volts).

 

to work out how much Current you want at that voltage will determine how many Zero go on the end of these value, rem that 1v across 1ohm draws 1 amp.

 

the rest should be easy

[scilearner]

Thanks for the help YT2095:-). Well in the answer to this question it says that volatage of Rb =8.3. Only way to obtain thi is 9-0.7 is 8.3 which is the VRb but what happened to VRa. Don't we have to substract VRa as well to get the VRb.

[YT2095]

Ra at .7 Ohm, and Rb at 8.3 Ohms will give your .7V at 1 amp to the base.

or 7 Ohms and 83 Ohms for the same voltage at 100ma etc...

 

although I am curious, is the Input waveform AC or running at +DC levels?

as to bias a transistor to Just on, isn`t good for AC waves, you need to hold it a little higher than only Just switched on, else you`ll lose anything less than .7v in the cycle.