# BJT basic amplifier problem

Discussion in 'General Electronics Chat' started by Abhinavrajan, Oct 6, 2016.

1. ### Abhinavrajan Thread Starter Member

Aug 7, 2016
77
1
Hi all.

Please have a look at the two attachments.

The first one is a biasing circuit.
In this circuit, I have to find the value of Rb1.
I have calculated that value which turns out to be 46.24k ohms.
But, it seems to be wrong.
I can't understand what mistake I am making.

The second ,attachment consists of a CE BJT amplifier.
I need to find the collector current and Midband gain of the amplifier.
Please tell me the correct method to approach to find the collector current and midband gain and help me solve it.

Thanks.

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2. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Since this is 'Homework Help' you are expected to show your efforts so far. Show how you got the value 46.24k.
Use Ohm's Law to calculate the circuit voltages and currents.

3. ### Abhinavrajan Thread Starter Member

Aug 7, 2016
77
1
Since, we know the value of Vb, Vcc and Rb2, I found the value of Rb1 using the voltage divider formula.

3.65=((12)(Rb1))/(105.8k+Rb1). I solved for Rb1 and found it to be 46.24k ohms.

4. ### Abhinavrajan Thread Starter Member

Aug 7, 2016
77
1
Please, explain a bit more clearer.

Thanks.

5. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Last edited: Oct 6, 2016

Aug 7, 2016
77
1
7. ### Veracohr Well-Known Member

Jan 3, 2011
549
75
You're given the emitter current & the transistor gain. That's all you need to calculate the DC base current.

Abhinavrajan likes this.
8. ### hp1729 Well-Known Member

Nov 23, 2015
1,938
218
When calculating the current through RB1 you need to subtract the base current from the current through RB2.

Abhinavrajan likes this.
9. ### Abhinavrajan Thread Starter Member

Aug 7, 2016
77
1
Thank you all. Got the answer!