# Class A amp with NPN BC547 transistor

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#### wess.vic

Feb 5, 2015
14
I'm having trouble making a simple amplifier. When the transistor turns on the output signal is lower than the original signal. Here's a picture of my circuit please help me I'm trying to learn transistors.

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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
try replacing the 11k resistor in the emitter of the transistor to something around 100 ohms and you might see an improvement.

#### Audioguru

Sep 24, 2016
3,656
There are many articles on the web about how transistors work. Yours is called "common emitter" and its voltage gain is roughly Rc/Re so your gain is an attenuation of 90/1000= 0.09 times. Your collector current is not 100mA, it is 1V/1k ohms= 1mA and the voltage at the collector is not 90mV it is 18V - (1mA x 90 ohms)= 17.91V.

If you use 10k ohms for Rc then the collector DC voltage will be about 8V and the gain will be about 10k/1k= 10 times.

#### Audioguru

Sep 24, 2016
3,656
Hi Steve,
The 11< is not 11k, it is 1k.

#### LvW

Apr 12, 2014
604
I'm having trouble making a simple amplifier. When the transistor turns on the output signal is lower than the original signal. Here's a picture of my circuit please help me I'm trying to learn transistors.

You are "making a simple amplifier". May I ask you if you have DESIGNED the shown circuit (calculation of parts values based on some requirements like quiescent current?). This would be the first - and necessary - step. Otherwise, you cannot understand any malfunction.

#### AnalogEL

Feb 21, 2017
11
Hi wess.vic,

From a quick look at your schematic it looks like you need to swap the collector and emitter resistors. If you do so, now you will get:

Emitter current Ie = (Vbe - 0.7V) / (90 Ohms) ~ 10mA
Collector current will be almost identical to this, so the voltage across the new Rc will be:

10mA * 1kOhm = 10V

With an 18V supply the voltage measured from the collector to ground will be:

18V - 10V = 8V, and the AC voltage gain will be roughly Rc / Re = 1kOhm / 90 Ohms ~ 11

The collector resistor and transistor will get a little warm with the bias current you've chosen, but the circuit should work just fine.

Let us know if it works!

#### Cannonball

May 6, 2017
193
Most of your input signal will go to the emitter and very little on the collector. Try putting a 22uf capacitor in parallel with the emitter resistor.

#### Audioguru

Sep 24, 2016
3,656
Most of your input signal will go to the emitter and very little on the collector. Try putting a 22uf capacitor in parallel with the emitter resistor.
Didn't you see the voltages?
The supply is 18V and the collector is 17.91V so the transistor is almost completely cutoff.
Adding a capacitor bypassing the emitter for AC will not correct the DC biasing error.
Change the emitter resistor to 8.2K to make it biased correctly then bypass some of the emitter resistor for some more AC voltage gain but with too much gain the top of the waveform will be squashed with severe distortion.

Oh, this amplifier certainly will not drive a low Impedance speaker.
Another Oh. This thread is almost 4 years old.

#### davenn

Moderator
Sep 5, 2009
14,240
Another Oh. This thread is almost 4 years old.

indeed it is

lets close it

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