Author Topic: Question regarding a simple amplifier circuit  (Read 2261 times)

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Offline sentryTopic starter

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Question regarding a simple amplifier circuit
« on: August 23, 2018, 05:47:26 pm »
Okay, so I understand that RE is used to stabilize the transresistance instability in the transistor to combat distortion by lowering the amplification gain for the DC signal. CE is used to bypass RE for the AC signal and thereby obtaining the full gain of the amplification, but only for the AC signal. What I don't understand is why this construction doesn't distort the AC signal because its unaffected by the negative feedback caused by RE? I would expect the AC signal to also be sensitive to the small hFE fluctuation caused by temperatur and current changes?

 

Offline David Hess

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Re: Question regarding a simple amplifier circuit
« Reply #1 on: August 23, 2018, 07:15:01 pm »
It does distort the AC signal to the extent that the transistor's own transconductance changes with current which changes with output voltage level.

This configuration is used to provide maximum AC gain while maintaining DC stability which would otherwise allow the transistor to suffer thermal runaway.
« Last Edit: August 23, 2018, 07:16:37 pm by David Hess »
 
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Offline Zero999

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Re: Question regarding a simple amplifier circuit
« Reply #2 on: August 23, 2018, 07:38:56 pm »
The hFE makes no difference to the transistor circuit's voltage gain, only its current gain. It's easy to build a transistor circuit to have a far higher voltage gain, than current gain.

RE is required because it would be virtually impossible to bias the transistor so roughly half the supply voltage appears across RC in its quiescent state, without it. VBE is too variable and the transistor has too higher transconductance to allow that to happen.

See link below for more information:
http://www.ee.ic.ac.uk/pcheung/teaching/aero2_signals&systems/transistor%20circuit%20notes.pdf

The distortion can be avoided, at the cost of reduced gain by removing CE or adding a resistor in series with it.
 
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Offline sentryTopic starter

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Re: Question regarding a simple amplifier circuit
« Reply #3 on: August 26, 2018, 04:36:53 pm »
Okay, so I constructed this simple amplifier and I was expecting to see the output signal phase shifted 180o from the input signal, but I only see 80o. Is that because of the output capacitor? I'm using a BC548 NPN transistor in my circuit.
 

Offline Zero999

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Re: Question regarding a simple amplifier circuit
« Reply #4 on: August 26, 2018, 05:55:37 pm »
Okay, so I constructed this simple amplifier and I was expecting to see the output signal phase shifted 180o from the input signal, but I only see 80o. Is that because of the output capacitor? I'm using a BC548 NPN transistor in my circuit.
It might be.

What frequency did you test it with and what's the load impedance?
 

Offline sentryTopic starter

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Re: Question regarding a simple amplifier circuit
« Reply #5 on: August 26, 2018, 06:20:19 pm »
Okay, so I constructed this simple amplifier and I was expecting to see the output signal phase shifted 180o from the input signal, but I only see 80o. Is that because of the output capacitor? I'm using a BC548 NPN transistor in my circuit.
It might be.

What frequency did you test it with and what's the load impedance?

I tested it with frequencies between 3 - 5 MHz. No load connected to the output, only my scope probe. Is that the reason, because I dont have a load connected?
 

Offline Audioguru

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Re: Question regarding a simple amplifier circuit
« Reply #6 on: August 26, 2018, 10:39:31 pm »
At the radio frequencies you are using, the electrolytic capacitors have inductance that causes phase shift and frequency response errors. Their values are way too high for radio frequencies, much lower value ceramic capacitors should be used.
Stray wiring capacitance also causes phase shift and frequency response errors at your radio frequencies.
If you are using a solderless breadboard then its wiring and rows of contacts have a lot of inductance and stray capacitance.
 
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Offline Zero999

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Re: Question regarding a simple amplifier circuit
« Reply #7 on: August 26, 2018, 10:48:48 pm »
Okay, so I constructed this simple amplifier and I was expecting to see the output signal phase shifted 180o from the input signal, but I only see 80o. Is that because of the output capacitor? I'm using a BC548 NPN transistor in my circuit.
It might be.

What frequency did you test it with and what's the load impedance?

I tested it with frequencies between 3 - 5 MHz. No load connected to the output, only my scope probe. Is that the reason, because I dont have a load connected?
That frequency range will be above the upper cut-off frequency of the amplifier, so the gain will start to fall by then and phase shift will be greater than 180o.

Another thing is the 'scope probe will present a capacitive load at that frequency, which will also add more phase shift to the circuit.

I suspect you have a phase shift of something like 440o.

Try 3kHz to 5kHz.

I think the biasing is also a little off. What's the collector voltage, when no signal is applied? It should be about  half the supply voltage, give or take a volt or so.
« Last Edit: August 26, 2018, 10:56:43 pm by Hero999 »
 
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Offline sentryTopic starter

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Re: Question regarding a simple amplifier circuit
« Reply #8 on: August 27, 2018, 07:48:54 pm »
Okay, so I constructed this simple amplifier and I was expecting to see the output signal phase shifted 180o from the input signal, but I only see 80o. Is that because of the output capacitor? I'm using a BC548 NPN transistor in my circuit.
It might be.

What frequency did you test it with and what's the load impedance?

I tested it with frequencies between 3 - 5 MHz. No load connected to the output, only my scope probe. Is that the reason, because I dont have a load connected?
That frequency range will be above the upper cut-off frequency of the amplifier, so the gain will start to fall by then and phase shift will be greater than 180o.

Another thing is the 'scope probe will present a capacitive load at that frequency, which will also add more phase shift to the circuit.

I suspect you have a phase shift of something like 440o.

Try 3kHz to 5kHz.

I think the biasing is also a little off. What's the collector voltage, when no signal is applied? It should be about  half the supply voltage, give or take a volt or so.

So I tried to lower the frequency to 3 - 5kHz. The result is much better now, especially when the signal is 4kHz. The phase shift is almost spot on now (180o). Excellent:D

Thank you for your help and suggestion.
 


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