# How can I design a proper BJT current amplifier?

Discussion in 'General Electronics Chat' started by HiThanks, May 26, 2019.

1. ### HiThanks Thread Starter New Member

Jul 10, 2018
2
0
In my project, I need to make current amplifier which have < 2mA input current and > 100 mA output current.
Actually, I want to fully understand the BJT circuit, and then making a proper amplifier circuit.
But project deadline is soon, so I just want to make the amplifier first.

present circuit with two TIP122 (NPN BJT) is follow,

My goal is applying the linearly amplified AC current (100~200 mA) in R_load.

The problem is ...
- Signal source have current limitation ( maximum current : 2 mA, maximum voltage : -10 ~ +10 V ).
- R3 have big DC current so, It is easily heated.
- non-linearly amplified...

So, can you suggest proper amplifier circuit for this situation?
If you give some circuit scheme with detail parameter ( Ohm, Volts, Capacitance, etc. ), would be perfect!

Thank you!

// summary
with 2 TIP 122,
- Input AC signal : -10 V ~ +10 V sine wave (current limit ~ 2mA).
- output AC signal : linearly amplified sine wave ( 100~ 200 mA )

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2. ### ericgibbs Moderator

Jan 29, 2010
7,709
1,553
hi HiThanks.
Is this a college assignment, if yes, I will move it to the Homework Forum, should improve responses.
E

3. ### ericgibbs Moderator

Jan 29, 2010
7,709
1,553
hi,
If you are trying to test a Current amp, you should use a current input source.
I will post a LTSpice sim of your circuit.
E

EDIT:
This is what I see with your circuit.

• ###### AA2 26-May-19 12.49.gif
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Last edited: May 26, 2019
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4. ### crutschow Expert

Mar 14, 2008
22,119
6,409
You need to AC couple the input signal with a series capacitor if want to amplify AC.

What is the purpose of R3 if you have a 5 ohm resistive load?
Do you want the load to see only AC?
If so, you need to AC couple the load also.

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5. ### Zeeus Member

Apr 17, 2019
325
10
with output cap, what happens to negative swing as cap discharges through R3 compared to charging through Rload

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6. ### crutschow Expert

Mar 14, 2008
22,119
6,409
It limits the magnitude of the negative swing. as compared to the positive swing.

Last edited: Jun 3, 2019
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7. ### HiThanks Thread Starter New Member

Jul 10, 2018
2
0
Thank you guys. I figured out.