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.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiMm7riP0RpIEST7_uVUQuE_H42u5b6XmLDKntkq1ILcdOHgYTUWvBtjL8_55yMG-OG9pXPt7BrAllKRy4BZAXhMWF2fVNSeEMTnRPW-sqhqCsxCtaSyGfT_XJrZf9MJg_8HHPKRPbxV-16/s640/10+times+Rc.jpg) |
Here I set Rl = 10Rc. 80mv output with 1mv input is reasonable. Look at Ic and RQ1. |
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiMQUjOByBTkGxae0_J4DZGf6358jtLey-YHjKn1xrVWUJDXyGW7x_t7uPQxEiHKkWhKDpZaGlExWlvVodKI1doArCOtODiuIAgHjcGPfZvmszTiSpPswPFVvrG6pWwbnuTxWTbLvIZNIac/s640/10+times+transistor+Z.jpg) |
I set Rl = 10RQ1 the power output increases. What's up with that? Vout is unchanged so it must be a current change? |
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhdQEDJhzyZ68l8Wy8tNN9fqhjZpwNphkXNVFnHG4S5QDKSbado8VxxeU5I8LH05i0Jo6owZNEvV8yGIIiDscnkVss4rxo3K3VJLCO025FtQwdxgb6EodhBX1fxN61YsS8RgsB7G4cz8jbR/s640/matching+Rc.jpg) |
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. |
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![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjUvqNQ341PgEmr6213_nwNIWJfEvRln-z-qJgmpxIhs7HtUmQT2dO1UhkF6StU3B763D3TdHXZvTtXCEBsu5VQiNprop4EmB4zpkdSfo35PDrqceGoJIIVNZvK0CAv6VRFt6XciIYll92q/s640/matching+transistor+Z.jpg) |
Wait a minute! setting Rl = RQ1 gives a higher power output than when Rl = Rc? |
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgII2yrRnEbOMrwudkk3TobmAVHkXdlH0c8MT6RTLfaouoWloXt_Y2Pw6ST2S4Sk53CT_n5_Qy0dkzhR2sikv4X2_3g_nhPAi4-8MyxTcFfuqjb5oF_JTOdVOz8NcErMSRnTlL2E7G-ehfl/s640/Z+matched+3.jpg) |
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. |
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEheFBqv47EJK-brfdU4t6K2TLHZwJYUa3vDB2mFV6ix_fokNHdDngcRjNgkYAinWxERUuJI-yzH2jZ2nFGIfmfQcBZdX93r2i1MxGNfmj6wPGCgfAqvym560gIL4D8OYHVajE6eqhwoX_9I/s640/Z+matched+4.jpg) |
In the previous circuit the transistor was dissipating half the power. This one is producing more output without heating the transistor as much. |
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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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