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how to do -1 with just transistor?

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ahgu

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How can I design a transistor amplifier that gives -1 amplification?
I want to generate a + and - input for briged amplifier configuration.

and I notice the distortion is very critical, I tried a common emiter configuration with Rc=Re=4.6k, and with biasing at the base for NPN 2n3903, it simulates fine in SPICE, but in real application, the output from Collection is always smaller. I really don't understand the logic since the IC should be very close to IE.

thanks
ahgu
 

source of error

The load you put on the amplifier stage will reduce the gain. The output impedance of the emitter side is very low and the collector is very high which explains the collector output being lower. You have two choices.
1. Increase the collector resistor to bring the level back up.
2. Put an emitter follower on the collector output.
 

I put a emitter follower and no difference

I put an emiter follower after the collect output and still no difference.

My Rc=4.7k , and the biasing at the emitter follower is 100k to 15v and 100k to ground. that gives me 4.7k||50k=4.3k, that is prabably the difference I have seen.

How can I bias the emtter follower so the input resistance is really huge?
The thing is the I dont want to adjust the Rc or Re for every circuit I am making.

and why the damn SPICE does not show the mismatch.

thanks
Ahgu
 

try this

Do not use a collector resistor for the emitter follower. For a NPN follower,

collector to positive supply
base to first stage collector
emitter to ground or negative supply through resistor.


One thing to check is if the first stage collector signal is the right value. Another is to check the DC bias conditions for the transistors to have Vce of a few volts.
 

-1 gain

well the whole purpose of this thing is to generate +1 and -1 output to drive a bridge amplifier. That actual power amplifier takes only positive input.

How can I invert the signal easily? I could use the original signal as one input, I need the other one 180 out of phase, but same maginitude!
-ahgu
 

you mean no base biasing?

you mean no base biasing? that way, the resistance it see is BRe=1Mohm . which is big enough.

also, I am amplifying audio signal.
 

inversion

The common emitter stage inverts the signal. The emitter follower on the output reduces the source impedance.

You would probably be better off with an op amp type stage.
 

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