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[SOLVED] Transistor as audio amplifier - no negative supply needed?

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lee321987

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Noob question here...

I've seen some audio amplifiers that use a standard BJT as the amp (example attached).

I'm wondering how this can work without a negative voltage supply.
I thought audio had to go above and below zero volts?

I guess it works, but are there common problems with such a circuit?

2 transistor audio amp.png
 

In circuits like this, AC is superimposed on DC, so the “0”V ac is somewhere around ½ DC, and it can go up to by ½ dc (to Vcc) and it can go down by ½ dc ( to “0V DC, or GND).

The separation is done by using capacitors between stages, which allow only AC signals to pass from stage to stage ..

:wink:
IanP
 
And your 2nd stage is an "A"-class amplifier, its idle power consumption is relatively high, so the transistor will be a bit hot:

- if the 2nd transistor has enough high h21 (beta) then its idle collector current will be appr 38 mA (4.5-0.7v on 100 Ohm)
and the idle dissipation is appr 200 mW ((4.5+0.7)V * 38 mA).

- I would reduce the value of the two base divider 100k resistors, eg to 10k.
 
The 100 k resister in the gate is actually providing a bias.. or say a DC offset to the input so as to compensate the effect of no - ive supply... this shift than can then be filtered out with cap at output to return to normal 0 level....
This is a common technique ( used in Opamp also.. to operate them with single +12 V) where u might don't have the output same as if u would have used a -ive supply but on the cost of less power consumption..
Google "Single supply for opamp" to have an idea
 
Both stages are class A amplifiers. In this mode an DC operating point or bias is required so the transistor never goes into cutoff or saturation when the AC input is present.
The first stage and second stages use different bias configurations. But they achieve the same goal. That is to stabilize the DC operating point against variations in the transistor gain (beta). The resistor at the emitter in both cases provides negative feedback and in some applications is bypassed by a capacitor to increase the AC gain.
To understand the disadvantages of class A amplifiers:

**broken link removed**
 
The first amplifier is the real amplifier it amplifies the Voltage of the signal to a large extent

The second amplifier is a class A amplifier which amplifies the power output of the signal by amplifying the current and maintaining the high voltage level got from amplification from the first amplifier

- - - Updated - - -

The first amplifier is the real amplifier it amplifies the Voltage of the signal to a large extent

The second amplifier is a class A amplifier which amplifies the power output of the signal by amplifying the current and maintaining the high voltage level got from amplification from the first amplifier
 
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