What is an Envelope Detector | How to Construct a Diode Envelope Detector

An envelope detector is an electronic circuit that demodulates the envelope of an amplitude modulated information contained in an AM signal. It is a non-coherent type of linear AM detector. These are used in radio receivers to detect or demodulate the original audio signals that are present in an AM signal. It is also called as the peak detector.

There are many different types of envelope detectors. The simplest, cheap and widely used envelope detector is the diode envelope detector. In diode envelope detector, the linear property of the diode is utilized for detection. It produces an output signal that follows the envelope of an amplitude modulated waveform, which constitutes the original modulated audio signal. There are many other types of AM detectors that are built into integrated circuits in some advanced radio receivers.

Working of a Diode Envelope Detector
The diode envelope detector consists of a detector diode, wave-shaping components and a low pass filter. The diode is operated in the linear region of its characteristic curve. The capacitor C and resistor RL form the wave shaping components or timing components that selects the time constant. The RLC network that follows it provides the low pass filter. The signal from the output of the low pass filter is amplified using an audio amplifier that drives a loudspeaker.

The amplitude modulated RF signal is given at the input of the diode detector. During the positive half cycle, the diode becomes forward biased and acts as a closed switch. It allows the positive half cycle of the radio frequency wave to pass through. The capacitor C charges to the maximum of the positive cycle.

During the negative half cycle, the diode becomes reverse biased and acts as an open switch, which blocks the signal through the diode. During this cycle, the capacitor discharges through the load resistor RL until the next positive half cycle.

Thus a waveform is traced from the envelope of the detected AM signal by the action of the diode, capacitor and the load resistor. This signal may contain some portions of the RF signal, which can be filtered using a low pass filter.

RC Time Constant

During charging, the RC time constant must be short.
RC << (shorter than) 1/fc

During discharging, the RC time constant must be long.
RC >> (longer than) 1/fc

RC must also be << (shorter than) 1/fm, the maximum modulating frequency
Therefore 1/fc << RC << 1/fm

Where RC is the value of resistor and capacitor of the time constant
fc - carrier frequency
fm - maximum modulating frequency

Distortions in Diode Envelope Detector
1# Creation of spikes: Due to the charging and discharging of the capacitor, spikes are generated. To minimize the spikes, the RC values must be kept high.
2# Negative peak clipping: In signals that are over-modulated or signals with more than 100% modulation, the values on the negative side will be clipped. This can be minimized by selecting the proper RC time constant.
3# Diagonal clipping: This occurs at high modulation depth. When the RC time constant is too long, it cannot follow the changes in the envelope of the demodulated waveform. This distortion can also be reduced by selecting the proper time constant.
 
Advantages of Diode Envelope Detector
1# It is a very simple and most commonly used AM detector.
2# It is very cheap.
3# It is efficient.
4# It is very effective for detecting narrow band AM signals.

Applications of Diode Envelope Detector
1# It is commonly used for detecting amplitude modulated signals in radio and communication receivers.
2# It is used for detecting double side-band full carrier signals.
3# It is used in electronic circuits to detect the presence of RF signals and its measurement.

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What is a Multistage Tuned RF amplifier | How to Construct a Multistage Tuned RF amplifier

An amplifier that uses multiple stages of tuned RF amplifier for amplification of an RF signal is called a multi-stage tuned RF amplifier. The individual single tuned RF amplifier stages are connected together in series by a coupling device such as a capacitor or a transformer. The output of the first amplifier stage is coupled to the input of the second stage. This type of joining together of two or more amplifier stages is called as cascading.

 

Types of Multistage Tuned RF Amplifier

There are several types of multistage tuned RF amplifiers and they are

1. Double tuned amplifiers: This is a transformer coupled amplifier. Here, both the primary as well as the secondary winding of the transformer is tuned. This type of amplifier has a wide bandwidth.
2. Stagger tuned amplifier: This is a tuned amplifier where two or more single tuned amplifier stages are tuned to a slightly different frequency. This amplifier has a wider bandwidth and it overlaps each other. It has a moderate gain.
3. Synchronous tuned amplifier: Here more than one single tuned amplifier is used and they are tuned to the same frequency. This amplifier has lower bandwidth and maximum gain.

Operation of a Multistage Tuned RF amplifier

The signal Vin that has to be amplified is given to the base of the first transistor Q1 through the capacitor C1. The transistor Q1 amplifies the signal and it is available at the primary winding of the transformer T1. The transformer T1 along with the parallel capacitor forms the LC tank circuit, which is tuned to the required frequency. The amplified signal from the collector is coupled to the input of the second transistor Q2 through the coupling capacitor C2.

The second stage provides further amplification of the signal and it is available at the primary winding of the transformer T2. The transformer T2 along with the parallel capacitor forms the LC tank circuit, which is tuned to the required frequency.

 

The amplified signal from the collector of transistor Q2 is available at the Vout through the coupling capacitor C3. The turns of the transformer can be adjusted so that the required coupling is maintained and thus maximum energy is transferred across the transformer windings.

The overall gain of a multistage tuned RF amplifier is determined by the formula

Gain of stage 1: Av1 = V2/V1

Gain of stage 2: Av2 = Vo/V2

Total gain of the stage is Av1 x Av2

Av = V2/V1 x Vo/V2 = Vo/V1
Av  = Vo/V1

Total gain of N number of amplifier stages is given by the formula
Av = Av1 x Av2 x AvN

Where Av = Overall gain
Av1 = Voltage gain of the 1st stage
Av2 = Voltage gain of the 2nd stage

Advantages of Multistage tuned RF amplifier

1. The voltage gain of a tuned RF amplifier is very high.
2. It has high impedance and high Q at its resonant frequency.
3. It has adjustable bandwidth depending on the type of amplifier that is used.
4. The current consumption of the amplifier is less.

Applications of Multistage Tuned RF amplifier

1. It is used as a multi-stage RF amplifier in communication receivers.
2. It is used as an IF amplifier in superheterodyne receivers.
3. It is used as an IF amplifier in television receivers and communication equipment.
4. It is used as an IF amplifier to convert higher frequency signals to a lower frequency in satellite receivers and communication equipment.

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What is a Tuned RF Amplifier | How to Construct a Tuned RF Amplifier

A tuned RF amplifier is a type of radio frequency amplifier that selectively amplifies a narrow range of radio frequencies from a wide frequency spectrum. This amplifier employs a tuned LC tank circuit in the place of its load. This tuned circuit is capable of selecting a narrow band of desired frequencies while rejecting all others. This process of selecting a specific narrow range of frequencies from a broad frequency spectrum is called as tuning.

Working of a Tuned RF Amplifier
The resonant frequency of an LC circuit is the frequency at which the reactance of the inductor balances with the reactance of the capacitor. It is denoted by Fr.

 

A parallel tuned radio frequency amplifier offers high impedance at the resonant frequency, and does not allow much current through it. It offers low impedance to all other frequencies away from the resonant frequency.

The tuned radio frequency amplifier offers high impedance at resonance that allows signals at the desired frequency, but offers low impedance to frequencies above or below the resonant frequency, thus rejecting signals in those frequencies. Thus a tuned amplifier selectively amplifies the desired signals and rejects all other signals.

The formula for calculating the resonant frequency of the amplifier is

Fr = 1/2π√LC

The Q factor or the sharpness of the resonance curve of the LC tuned circuit of the amplifier determines the selectivity of that circuit. The Q factor of a resonant circuit is dependent on the internal resistance of the inductor (L) of the circuit. The lower the resistance of the inductor, the higher the Q factor.

 Tuned Radio Frequency Amplifier Circuit

 

Advantages of Tuned RF Amplifiers
1# The use of reactive components in a tuned radio frequency amplifier such as the inductor and the capacitor reduces the power loss at resonant frequency due to the high impedance which makes it efficient. Thus a smaller collector supply voltage is necessary.
2# The selectivity and amplification of the desired radio frequency signal is high in tuned radio frequency amplifiers.

Applications of a Tuned Radio Frequency amplifier

1. A tuned RF amplifier is used as a front-end radio frequency amplifier in radio, television and communication receivers.
2. It is used as a tuned Intermediate frequency amplifier in radio, TV and communication receivers.
3. Tuned amplifier is used as a selective RF amplifier to amplify only the desired signals in transceivers and transmitters.
4. RF power amplifiers utilize tuned RF amplifiers to amplify the required signals before they are being transmitted through the antenna.
5. Tuned amplifiers are used in frequency converters and doublers to selectively amplify the required signals while rejecting all others.

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What is a Hartley Oscillator | How to Construct a Hartley Oscillator

A Hartley oscillator is an electronic LC oscillator that consists of an amplifier and a feedback circuit whose frequency is determined by the LC tank circuit. It is also called the split inductance oscillator. This circuit was invented by Ralph Hartley in 1915. In this oscillator, the tuned circuit consists of a capacitor which is connected in parallel, with two inductors that are connected in series. The two inductors are connected together at the center as a center-tapped inductor.

Two Types of Hartley Oscillator 

1. Series-fed Hartley oscillator: This is not commonly used because of its frequency instability.

2. Parallel or shunt-fed Hartley oscillator: This is highly stable and it is more commonly used.

Working of Hartley Oscillator
When a voltage VCC is applied, it crosses the radio frequency choke or RFC and increases the collector current. The RFC at the collector provides high reactance to higher frequencies and behaves as an open circuit. At DC voltages, it produces a low reactance condition and acts as a short circuit thus allowing the DC to pass through. The collector current signal reaches the tank circuit through the output coupling capacitor C2. It charges the capacitor C of the tank circuit and the energy is stored as an electrostatic field.

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When the capacitor C is fully charged, it then goes into discharge and this discharge reaches the inductor L1. The L1 gets charged and the charge is stored as a magnetic field. When the L1 is fully charged, it then goes into discharge and this discharge returns back to the capacitor C and charges it. This back and forth charging and discharging goes on between the capacitor and the inductors which forms a sine wave. It changes its direction each time it charges and discharges. This sine wave is less intense and has a negative alternation.

The mutual induction between the L1 and L2 transfers energy from the collector circuit to the base of the transistor. The oscillations at the L1 are transferred to L2 and this causes a 180 degree phase shift. The energy from the tank circuit is fed directly to the base of the transistor through a coupling capacitor C1. The NPN transistor with a common emitter configuration amplifies the signal and inverts it into 180 degrees. The oscillations at the LC tank circuit and the output are in phase.

 Hartley Oscillator Circuit

 

Advantages of the Hartley Oscillator

1. The Hartley oscillator needs few components at the LC tank circuit.
2. The amplitude of the signal is constant over a wide frequency range.
3. The frequency may be varied by changing the value of the capacitor or the inductor.

Disadvantage of the Hartley oscillator
The sine-wave of the Hartley oscillator contains many harmonic components thus pure sine-wave cannot be obtained.

Calculation to find the Frequency of Oscillation of a Hartley Oscillator

The frequency of oscillation of the Hartley oscillator is calculated by the formula

F = 1/2π√LT C,

Where LT = L1 + L2 + 2M

2M is the mutual inductance between the two inductors

Applications of Hartley Oscillator

1. It is used as a local oscillator in radio receivers.
2. It is used in frequency converters and doublers.

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