# Tuned circuits

Discussion in 'General Electronics Chat' started by froten_140, Aug 5, 2008.

1. ### froten_140 Thread Starter Active Member

Jun 4, 2006
30
4
Hello all,

When designing a tuned RF amplifiers, a LC circuit is used in the input of the amplifier or as a load of its transistor collector. The L&C values for a certain frequency can be changed for the same rfrequency. For Example, for 4Mhz, we can use 1uH with 1583 pf, 10uH with 158 pf or another values.

What is the values selection trick?

2. ### Wendy Moderator

Mar 24, 2008
21,778
3,024
What varies is the Q of the tank circuit. In most ocsillators I don't think it makes much difference, but for filters it determines the bandwidth.

Apr 5, 2008
19,699
4,060
4. ### studiot AAC Fanatic!

Nov 9, 2007
5,003
522
In any transistor circuit where the output is from the collector, the transistor wants to drive the same collector current, regardless of the impedance of the collector load. It takes the appropriate collector voltage to cause this to happen, allowing the rest of the supply to be dropped across the load.

It follows that the higher the load impedance the higher the voltage dropped across it to maintain this state of affairs.

Thus if we want to maximise the output voltage across the load we must maximise its impedance.

Thus we go for as high value resistor as we can or use some special characteristic of another sub circuit to achieve high impedance.

One method is to use current generators as active loads. this is often done in integrated circuits.

However the tank circuit, comprising a parallel inductor and capacitor, has the very desirable characteristic of having its impedance vary with frequency, with a very pronounced maximum at the frequency of interest. So it will automatically 'tune' the overall voltage output (GAIN) of the circuit to this desired frequency.

As to which combination of L and C, there is no magic. It just depends what you have that will minimise the resisitve element of the tnak. This is because the larger the ratio of the reactive impedance to the resistive (called circuit Q factor) the better.

The series LC circuit, of course, has an impedance minimum at the frequency of interest.

5. ### iamspook Member

Aug 6, 2008
27
0
This becomes more critical as the frequency goes up.
At some point, wires and traces connecting the components
look like significant values in the calculation, and this is where
careful spice analysis is best. But even before that, any lumped
component considered ideal at low frequencies will actually
have resistance, inductance and capacitive "flaws".
For a coil, it has a reasonably measurable or calculable series
resistance which will affect the Q factor. A capacitor will have
an equivalent series resistance and leakage which will start to
affect the circuit. As mentioned, when you take these into consideration,
choose that which gives a desired result. This may be based
on :

* Cost
* reliability
* availability
* physical size
* ease of construction
* temperature considerations
* power handling

etc

If you don't care about some of the above or the influence of the
flaws is insignificant, then there will be multiple suitable choices.