RF Amplifiers

Processes the very weak input signal. Selectivity such that it eliminates imagesEssential to use low noise components.For lower frequency (<30MHz) no need of RF amplifier. Only use tuned circuit for selectivity; image rejection. Thus, mixer this kind must be low noise.For frequency above 100MHz, usually have RF amplifier, to increase the weak signal prior to mixing. A single transistor can give gain 10-30dB.For lower freq, use bipolar transistors. For VHF, UHF and microwave, use FETs. FETs have lower noise characteristics-give better performance.

a) Broadband bipolar RF amplifier Antenna connected directly to base of transistor. Circuit is broadband; the transistor can pick up and amplify any incoming signal.At collector, has tuned circuit for selectivity before going to mixer.

b) Tuned FET RF amplifier Have tuned circuit in front. Thus, Q is higher, and better selectivity. Also have low noise figure.

Mixer and Converters

Circuits to perform mixing or heterodyning or frequency conversion.Up conversion-translating the input signal to a higher frequency.Down conversion-translating the input signal to a lower frequency.4 types of mixers usually found in communication:

a) Diode mixersb) Doubly Balanced mixersc) Transistor mixersd) IC mixers

Diode mixers

Most common type found in microwave applications.Input is applied to primary winding of transformer T1. The signal is coupled to secondary winding and applied to the diode mixer. The LO signal is coupled to the diode via C1.The output signals are: Fs, F0, Fs+ F0, and Fs-F0/F0 -Fs.Then, developed through the tuned circuit to select the frequency desired.For small signal applications, use germanium diodes (low turn on voltage).For VHF, UHF, and microwaves, usually hot carrier or Schottky barrier diodes.

Doubly Balanced Mixers

Balanced modulators are widely used as mixers. Advantage: carrier eliminated from output, thus easier in designing filters. Diode lattice balanced modulator, and differential amplifier type of balanced modulator can both effectively be use s mixers. One version that is most popular in VHF and UHF applications is Doubly Balanced mixer.

Transistor Mixers

Transistors, if biased to a nonlinear region, can be use as mixers to perform analog multiplications.Advantage over diode is that the gain is obtained in the transistor stage.Examples: bipolar transistors mixers-used at low frequencies; junction FETs and Dual-gate MOSFETs (provide superior performance in mixing applications) at VHF, UHF and microwaves because of their high gain and low noise. Gallium Arsenide (GaAs) is preferred rather than silicon at high frequencies also because high gain and low noise.

IC MixersKnown as NE602.Consists of double-balanced mixer cct made up of two cross-connected differential amplifiers.

IF Amplifiers

This is where most of gain and selectivity is obtained.In designing receiver, the choice of IF must compromise between good selectivity and stability; and good image rejection.Notes: remember good selectivity when IF is low frequency, and good image rejection when choosing a high IF.For AM usually use 455 KHz;-low enough to get good selectivity and make high gain with minimum instability.However, for input frequency above 10MHz, higher IF is selected, because it is not anymore sufficient to remove image rejection.For 30MHz, usually use IF=1500-2000 kHz.For most FM receivers (88MHz-108Mhz) we use 10.7 MHz of IF frequency.Usually some receivers also use double conversion to solve the image and selectivity problems.The selectivity of the IF amplifiers is provided by the tuned circuit. Cascading the tuned circuits causes the overall circuit bandwidth to narrow. In designing IF, make sure the selectivity is not too sharp that will cause sideband cutting. This means that the higher frequency amplitude will be greatly reduced in amplitude thus distorting the signal. In some cases it’s necessary to widen the bandwidth when we get a broadband signals.Two ways of doing so is: 1st by connecting a high value of resistance across the parallel tuned circuit, thus lowering the Q that will produce the appropriate bandwidth.Another way is to use a double-tuned transformer.Principle: the spacing between the primary winding and secondary windings determines how much the magnetic field produced by the primary will cut the turns in the secondary. Thus, the farther away the spacing of the coil, the amplitude will be lower and the bandwidth will be narrower.Define under coupling, critical coupling, optimum coupling and over coupling.Thus, by setting the amount of coupling between the windings in the IF coupling transformer, the desired amount of bandwidth can be obtained.

Two stage IF Amplifier using double tuned transformer coupling for selectivity

Ceramic and crystal Filters

In most communication receivers where superior selectivity is desired, very sharp ceramic and crystal filters are use to obtain the desired selectivity.Ceramics can be cut and shaped so that it is sharply resonant over a narrow band of frequencies centered at the resonant frequency.Since it is design for specific frequency, ideal to use for IF selectivity. Thus, the IF amplifiers are only broadband circuits. The filter itself has set the bandwidth.

Automatic gain control

AGC is a feedback system that automatically adjusts the gain of the receiver based on the amplitude of the received signal.Very low level signal will cause the gain of the receiver to be high. Very high level signal will cause the gain of the receiver to be reduced.Having the AGC, the receiver can have a very wide dynamic range.Dynamic range refers to the measure of the receiver’s ability to receive both the very strong and weak signal without introducing distortion. Typically its

60-100dB range.

How does it works?

AGC take the received signal from o/p of IF amplifier or demodulator and converts it to DC. The amplitude of DC is proportional to the level of received signal.The dc is applied to one or more stages of IF amplifier to control their gain.Note that the gain of bipolar transistor amplifier is proportional to the collector current.Increasing Ic will increase the gain. However, at some point, the gain will flatten over a narrow Ic gain, then begins to decrease. Thus, increasing the IC after this point will decrease the gain.Thus, the gain of the amplifier can be adjusted in two ways,by increasing or decreasing the collector current.

Reverse AGC

An AGC circuit that decreases the current flowing in the amplifier in order to decrease the gain is called reverse AGC.In circuit shown, the bias of the amplifier is derived by voltage divider R1 and R2, and emitter resistor R3.R4 applied to the base will accept the negative DC from AGC. As signal amplitude increases, the negative DC voltage increases, thereby decreasing the base current. Thus, this decreases the collector current, and lowers the circuit gain.

Forward AGCThe AGC that increases the current flowing in the amplifier in order to decrease the gain is known as forward AGC.The bias in this circuit is derived only by R1 and the AGC circuit itself.The DC voltage is positive, which sets the bias level.As the signal amplitude level increases, the DC voltage will increase.This in turn, increases the base current, increasing the collector current.Thus, the gain will be reduced.

AGC in AM

Diode detector recovers the original signal.Voltage across R1 is a negative dc voltage. C1 filters out IF signal, leaving only original signal.The recovered signal is applied to C2 to remove the DC. The resulting ac is amplified by audio amplifier and applied to loudspeaker. The DC voltage across R1 and C1 are filtered out further to get pure DC. This is done by R2 and C3. Note: must make sure the time constant of the component be so large that the voltage output is pure dc.The dc level will vary with the amplitude f the received signal.The resulting negative signal will then be applied to one or more IF amplifier stages.

Squelch circuit

Also known as mute circuit.Designed to keep the receiver audio off until an RF signal appears at the receiver’s output.Provides a mean of keeping the audio amplifier turned off during the time noise are heard in the background, and enabled the audio when a RF signal is received.Principle of popular types of squelch circuit: amplifies the high frequency background noise and use it to keep the audio turned off. When signal is received, the noise will be overridden, and the audio will be turned on.

Notes:Most two ways comm are short conversations, and not always continuous. However the receiver is left on so that if a call is to be received, it will be heard.Thus, when no RF signals in the input, the audio output is just background noise, which could be annoying.Usually, people will just turn down the volume, but then this will cause the RF signal not to be heard.

A noise derived squelch circuit

The background noise with no F input is taken in one of the IF amplifier.Passed through C1 and R1 that acts as a high pass filter. (R1 also acts as the squelch control level).The noise is amplified, then rectifies to dc voltage by rectifier-voltage doubler.The output of the rectifier will cause the squelch gate Q1 to saturate. The base current of audio amplifier Q2 is shunted away through Q1. Thus, no audio amplification takes place and the receiver is quiet.If a voice signal occurs, it will blank/mask any high frequency noise.Thus no noise signal will be applied to the squelch circuit.Notice that high pass filter C1 and R1 will not pass through voice signal since it is below 3kHz. Thus, no squelch voltage applied to Q1.Q1 cuts off. Q2 is biased normally and the audio signal is passed through the audio power amplifier and the speaker.