what is the usage of power amplifiers in these circuits?

[phoibos323]

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what is the usage of power amplifiers in these 3 circuits? Thank you.

 

[pakitos]

It is a power amplifier that is it amplifies both current and voltage = power.
You can measure the current before and after.
Remember you have to out the ammeter in series.
If these are not sufficient scan the characteristics of the outputs i.e. buzzer table 15...
Good luck

 

[cascoder]

Hi phoibos323,

the power amp in these circuits basically takes a voltage at its input, multiplies this voltage by a constant (called the "gain") and then puts this new voltage on its output. It can supply this new voltage into a load of low resistance ie: it can supply a rather large amount of current at its output; while at its input it draws very little current ie: it presents a high resistance to the potentiometer, so it does not overload the potentiometer.

The distinction between "power amp" and "op-amp" is just that power amps are able to drive more current than op-amps; power amplifiers can supply typically from 100mA to 10A (depending on their design and how big their output transistors and heat-sinks are), while op-amps are just little chips and can only supply 10mA typically. I suspect that the solenoid in figure 18 needs more than 10mA to operate properly...

I suspect that in these questions, the gain of the amplifier is just 1, that is, the power amplifier is being used as a "buffer" that takes a voltage at its input drawing very little current, and puts the same voltage at its output and can drive any current that the load connected at the output needs. For example, in Figure 18, the potentiometer could not supply the current that the solenoid needs to operate; the solenoid has a low impedance, and the potentiometer has a high impedance; if you connect terminal B directly to the solenoid, it would not matter what position you put the potentiometer in, the solenoid voltage will remain at zero and it will not work (well, actually that is not entirely true, as the wiper of the pot moves closer to point C you will get some voltage at the solenoid, but you will probably also burn-out the pot!).

The amplifier therefore is used to "buffer" the output of the pot; the pot is used to create a smoothly variable DC voltage, and the amplifier puts this voltage across the solenoid.

Hope this helps.