Amplification, Oscillators, Feedback



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How Does an Amplifier Work?

Consider this diagram. 12v dc is applied across two resistors R1 and R2 in series.

Amplification

Point A will therefore be at 6 v.

The meter will read + 6v.

The lower resistor R1 is variable.

If we increase the value of R1 to 120k, we can calculate the voltage at point A.

If the value of R1 is reduced to 80k, we can also calculate the voltage at point A.

So if the value of R1 is varied, the reading on the meter will decrease or will increase.

Those variations, (in effect an alternating signal), will be passed via the coupling capacitor to the next stage - shown by the arrow to the right. We can replace one variable resistor with another. Consider R1 to now be replaced with any one of the devices shown in this following diagram. Each can act as a variable resistor:

The connections to these devices to replace R1 are shown. (The thermionic devices will require more than a +12v supply - but the principles still apply.)

Devices

Other connections must be made to the other electrodes of each device to bias it for correct operation so that the input signal can vary the bias which alters the internal resistance of the device, which: in turn varies the voltage at point A, which is passed on through the coupling capacitor to the next stage for further processing.

The upper resistor in the above diagram, R2, is known as the load. This can take other forms as the following diagram shows.

AFamp

Here the load is an audio transformer with a loudspeaker connected to its secondary.

The two resistors connected to the base of this transistor with the resistor in the emitter, form the biasing arrangements. These components set the level of the current flowing through the primary of the transformer - the collector current.

An input audio signal is fed via a coupling capacitor to the base of the transistor. This varies the base current which in turn varies the effective resistance of the transistor and hence the collector current.

Only small changes in the base current are needed to make much larger changes in the collector current - amplification!

The circuit below is a typical transmitter radio frequency amplifier. Here the load comprises all the components connected to the collector of the transistor Q1 - C5, L2, etc. and the antenna or whatever is connected to the output. The RF choke RFC2 feeds 12v DC to the device and prevents the RF from getting into the power supply leads. The input signal provides self-bias to the transistor.

RFamp

Positive Feedback - and Oscillators

If you take part of the output from an amplifier and feed it back to the input, provided some special conditions are met, the device will oscillate.

RFosc

This means that it generates a signal. The frequency of the signal depends on the circuit component values and the feedback arrangements.

The conditions for oscillation are that the level of signal fed back is at an adequate level, and that the signal is in the correct phase to sustain oscillation. This is positive feedback.

This means that the signal fed back adds to the signal at the input to the amplifier.

This diagram shows a simple radio-frequency oscillator. A tuned circuit in the collector circuit sets the frequency of oscillation.

The feedback is taken by a secondary coil and inserts a signal in the base lead, changing the base-current. Provided the secondary coil is correctly polarised, the circuit will oscillate.

This next diagram shows an audio amplifier (the triangle - with gain in the direction of the arrow) and a feedback network - the collection of resistors and capacitors - a bridged-tee network - connected between the output and the input.

AFosc

This again is a diagrammatic illustration of an audio oscillator. There are many different feedback networks used and they can comprise a wide range of components of all types.

There are many different oscillator circuit types, as reference to a textbook will show!

How Does it Start?

Osc1

In practice, when first switching on, an oscillator will usually self-start because a burst of noise or a similar transient at the input to the amplifying device is enough for it to commence oscillation.

Oscillators can usually be identified because they have an output with no input shown - other than the DC supply.

This diagram is also an oscillator. The bottom end of the coil L1 is common to the source and drain current path and transformer action will cause changes in the base current. This device is self-biasing.

Osc2

This circuit is a crystal oscillator. A quartz crystal can be regarded as a high-Q tuned circuit. Resonance

The quartz crystal is shown with two capacitors across it to provide the feedback for oscillation. .

A resistor from the collector to the base and from base to earth, together with the emitter resistor, provides some DC bias (base current) for correct operation.

Compare this diagram with the previous one.

The Voltage-Controlled Oscillator (VCO)

VCO

This circuit is the same as one shown before with some components added.

A voltage-controlled oscillator is one in which the frequency of oscillation can be varied by changing a voltage applied to it.

Diodes D1 and D2 are varicap or varactor diodes connected across the tuned circuit L1 and C1.

When these diodes are reverse-biased, the depletion region between anode and cathode becomes a dielectric whose width is dependent on the applied voltage. A change of applied voltage changes the width of the dielectric thereby changing the capacitance between anode and cathode.

Connected as shown, changing the voltage at the wiper (moving arm) of the manual TUNING potentiometer shown will change the frequency of the oscillator. The 100k resistor at the junction of the two diodes is to prevent any RF from entering the DC line.

The Frequency Synthesiser

Synthsiser

A saving in the number of crystals needed for switched-channel equipment, can be made by using a synthesiser.

This diagram shows two oscillators each with a selection of crystals.

A mixer combines these outputs to provide other frequencies by using the sum (or difference) outputs from the mixer. Mixers

A wide selection of channels can be provided for a transceiver by this method.

The Phase-Locked Loop

This diagram shows the principle of the phase-locked loop (PLL). It consists of a voltage-controlled oscillator which provides the output frequency. That frequency is compared to a reference oscillator using a phase detector or comparator.

PLL

A sample of output from the VCO is passed through a frequency-divider stage to the phase detector.

The phase detector supplies an error voltage to the voltage-controlled oscillator to keep it accurately on frequency.

If we want the output frequency to be the same as the reference oscillator we pass the output frequency through a divide-by-one stage to the phase detector. If the frequencies are not the same, an error voltage proportional to the difference in frequency is produced. This voltage is filtered and applied to the VCO to bring it back on frequency.

If we want a frequency 10 times the reference frequency, we tune the VCO to this frequency. The output is then passed through a divide-by-10 stage to the phase detector which operates as in the previous case.

When we want a frequency 20 times the reference frequency, we divide the output by 20 and apply it to the phase detector.

Modern transceivers and other equipment use the phase-lock loop principles and can tune in 1 kHz and often smaller, steps.

The PLL as a Demodulator For FM Signals

PLL principles can also be used as a demodulator in an FM receiver.

PLLdemod

The loop locks on to the input signal and the VCO will follow the instantaneous frequency of the input signal.

Variations in the input frequency are converted into variations in the loop control voltage.

The control voltage must change and it is this voltage that corresponds to the demodulated signal, the audio output. A buffer is used to isolate the output circuitry from the control loop.

Negative Feedback

Negative feedback is a signal fed back to the input of an amplifier so that it opposes the input signal - the opposite of positive feedback. It does have great advantages in some applications, in particular in hi-fi audio amplifiers. For amateur radio purposes, there is one useful application - the emitter-follower circuit (or the cathode-follower circuit).



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Compiled Thu Jan 20 2011 at 9:07:41pm


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