RF/IF FILTERS/TRANSFORMERS

RF/IF FILTERS

These filters can be found in radio receivers and act a an Inductor, Capacitor and Transformer in one package.  These are specially designed tuned circuits.
The resonant frequency can be adjusted by tuning the coloured ferrite core.
In an ordinary radio, you will most often find 4 types of IF-cans. For the FM part the IF frequency is 10.7MHz.

The color of the slug in this CAN is most often pink. For the AM part the IF frequency is 455kHz.

RED - Oscillator. With 30pf - 300pf = 1MHz to 2MHz
YELLOW - First 455KHz IF filter transformer
WHITE - Second 455KHz IF filter transformer (not always used)
BLACK - Last 455KHz IF filter transformer

The can with the PINK color is a quadcoil for the FM demodulation.
This can is a LC unit tuned to 10.7MHz

Note the frequency of 455 or 10.7MHz.  Within this is the bandwith, which can be 150kH, 180kH ,220kH etc

INCREASING THE FREQUENCY OF AN AMPLIFIER

VIDEO FREQUENCY AMPLIFIERS

To increase the frequency bandwidth of an amplifier, an inductor is introduced into the circuit.  As we know, increase in frequency causes more EMF to develop across an inductor which enables a greater load to develop across the inductor.  The inductor acts as a store of the energy.

The bandwith of an amplifer is limited in a circuit because of the internal capacitance of the circuit and the capacitance of the amplifer (transistor).  

In the first case, the capacitance usually shunts the signal to ground.  As frequency increases, the capacitance acts like a short and most of the signal is shunted to ground.
In the second case, the capacitance of the transistor causes the base and the emitter to increase at the same time causing the transistor to not amplifer the signal as much.

There are two types of methods to increase the frequency response of a transistor amplifer.  One is my using Shunt Peaking, whereby an inductor is placed on the output (collector) of the amplifer in parallel to the output signal path (but in series to the load (signal) usually a resistor), and the other is by Series peaking, by placing a coil on the output stage, but in series with the output signal.

MORE ON IMPEDANCE

If a device has a HIGH INPUT IMPEDANCE, that means low current will flow which in turn means that the device requires low current to work!  Take for example the following statements

The FET input means that the op-amp has a very high input resistance and only needs a tiny input current to work. This means it is easy to ‘drive’ - i.e. the signal source doesn't have to supply much current. The Bipolar output means that the op-amp can provide just as much output current as an all-bipolar design could.

source:http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/datasheets/Opamps/071.html

IMPEDANCE MATCHING

If maximum current is desired at the input to a circuit, should the input impedance of that circuit be lower than, equal to, or higher than the output impedance of the previous stage?


Answer:  The circuit should have a lower impedance compared to the circuit the circuit of the previous stage.  This will allow more current to flow given the lower resistance.

What transistor circuit configuration should be used to match a high output impedance to a low input impedance?

Answer:  The question is thus: Circuit A has high output impedance - MyCircuitB - Circuit C has a low input impedance
Hence MyCircuitB should have a Common Collecter configuration since a Common Collector configuration provides high input impedance and low output impedance.

Low Feeds High
In order to preserve signal level and frequency response, it's important
to drive equipment with a source signal that is lower in impedance than
the destination equipment's input impedance. If the input impedance of a
device is not significantly higher than the source impedance, the signal
will be reduced or "loaded down" and its signal to noise ratio and
frequency response will suffer.

Think of this as having a nozzle at the end of a garden hose. The garden
hose is a low impedance source (there is little resistance to the flow
of water) and the nozzle is the higher impedance of the input being fed
by the hose.

When the nozzle valve is closed (open circuit):

* Input impedance is VERY high
* Pressure (voltage) is at maximum
* Flow (current) is zero

Now open the nozzle just a little:

* Input impedance reduces but remains high
* Pressure reduces but remains high
* Flow is small

As you continue to open up the nozzle:

* Input impedance reduces further
* Pressure reduces
* Flow increases


With the nozzle open all the way:

* Input impedance is very low
* Pressure falls dramatically
* Flow is greatest


In the case of a high impedance guitar output (7,000 to 15,000 Ohms or
more) driving a relatively low impedance input of a mixer (2,000 to
10,000 Ohms), it's like connecting a garden hose to a fire nozzle. The
hose just can't produce enough flow (current) for the size of the
opening (impedance) to maintain the pressure (voltage).

If the load impedance (nozzle/FET) is 10 times or more the source
impedance (hose/AC Signal), it is called a "bridging" impedance.
Bridging results in maximum VOLTAGE transfer from the source to the
load. Nowadays, nearly all devices are connected bridging -- low-Z out
to high-Z in -- because we want the most voltage transferred between
components.