Tuesday, April 30, 2019

power consumption vs power out. vs voltage out

The "rule of thumb" is that the output of an amp is equal to Rc. If this is so then making Rl = Rc will give the Z match for maximum power out. We need to consider is the transistor a voltage amp or a current amp. You can look at it as a voltage controlled resistor OR a voltage controlled current source. We need to consider the frequency and the need for stability too. I generally like to make low voltage and low power circuit to operate on battery power. This influences the design too. First a series of screen shots to consider.
Here I set Rl = 10Rc. 80mv output with 1mv input is reasonable. Look at Ic and RQ1.

I set Rl = 10RQ1 the power output increases. What's up with that? Vout is unchanged so it must be a current change?

I set Rl = Rc and get the higher output.  It appears making Rl = Rc gives a higher power output and making Rl = 10Rc gives a higher voltage.

Wait a minute! setting Rl = RQ1 gives a higher power output than when Rl = Rc?

I set Rl = Rc / 10 and raised Ic to 10ma. More power out. Note Rl is about 1/10 Rc. Look at RQ1. So it is becoming more obvious that for voltage gain we want a high Z circuit and for power gain we want a lower Z circuit.

In the previous circuit the transistor was dissipating half the power. This one is producing more output without heating the transistor as much.

How much Ic do we want. The last two shots have the same power out BUT one is 10ma and the other 50ma.
Some food for thought?

Edit: Look at some RF 50 Ohm amps and you will find most run at about 10 ma Ic. This seems to be the compromise for Z matching and power consumption.
Audio systems were designed for 600 Ohm allowing lower currents and power consumption.
At RF the lower Z helps overcome circuit reactances. In a microwave circuit a wire running a fraction of an inch above the ground plane will have inductance and capacitance  and the circuit is more critical. I'm working in the HF range which is more forgiving.
As I said food for thought.

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