IF Amplifiers | RF Input Amplifiers | Squelch Circuits | Controlling Gain

Typical Receiver Circuits

IF Amplifiers : This section focuses on RF and IF amplifiers, AGC and AFC circuits, and other special circuits found in receivers.

RF Input Amplifiers

The most critical part of a communication receiver is the front end, which usually consists of the RF amplifier, mixer, and related tuned circuits and is sometimes simply referred to as the tuner. The RF amplifier also called a low-noise amplifier (LNA), processes the very weak input signals, increasing their amplitude prior to mixing. It is essential that low-noise components be used to ensure a sufficiently high S/N ratio. Further, the selectivity should be such that it effectively eliminates images.

In some communication receivers, an RF amplifier is not used, e.g., in receivers designed for frequencies lower than about 30 MHz, where the extra gain of an amplifier is not necessary and its only contribution is more noise.

IF Amplifiers | RF Input Amplifiers | Squelch Circuits | Controlling Gain

In such receivers, the RF amplifier is eliminated, and the antenna is connected directly to the mixer input through one or more tuned circuits. The tuned circuits must provide the input selectivity necessary for image rejection. In a receiver of this kind, the mixer must also be of a low noise variety. Many mixers are MOSFETs, which provide the lowest noise contribution. Low-noise bipolar transistor mixers are used in IC mixers.

Most LNAs use a single transistor and provide a voltage gain in the 10- to the 30-dB range. Bipolar transistors are used at the lower frequencies, whereas at VHF, UHF, and microwave frequencies FETs are preferred.

The RF amplifier is usually a simple class-A circuit. A typical FET circuit is shown in Fig. 9-27. FET circuits are particularly effective because their high input impedance minimizes loading on tuned circuits, permitting the Q of the circuit to be higher and selectivity to be sharper. Most FETs also have a lower noise figure than bipolars.

At microwave frequencies (that above 1 GHz), metal-semiconductor FETs, or MESFETs, are used. Also known as GASFETs, these devices are junction field-effect transistors made with gallium arsenide (GaAs). A cross-section of a typical MESFET is shown in Fig. 9-28. The gate junction is a metal-to-semiconductor interface because it is in a Schottky or hot carrier diode. As in other junction FET circuits, the gate-to-source is reverse-biased, and the signal voltage between the source and the gate controls the conduction of current between the source and drain. The transit time of electrons through gallium arsenide is far shorter than that through silicon, allowing the MESFET to provide high gain at very high frequencies. MESFETs also have an extremely low noise fi gure, typically 2 dB or less. Most MESFETs have a noise temperature of less than 200 K.

IF Amplifiers | RF Input Amplifiers | Squelch Circuits | Controlling Gain

As semiconductor processing techniques have made transistors smaller and smaller, both bipolar and CMOS LNAs are widely used at frequencies up to 10 GHz. Silicon germanium (SiGe) is widely used to make bipolar LNAs, and BiCMOS (a mixture of bipolar and CMOS circuits) designs in silicon are also popular. Silicon is preferred because no special processing is required as with GaAs and SiGe.

Although single-stage RF amplifiers are popular, some small-signal applications require a bit more amplification before the mixer. This can be accomplished with a cascade amplifier, as shown in Fig. 9-29. This LNA uses two transistors to achieve not only low noise but also gains of 40 dB or more.

Transistor Q1 operates as a normal common source stage. It is directly coupled to the second-stage Q2, which is a common gate amplifier. The gate is at ac ground through C3. The frequency range is set by the input tuned circuit L2 – C1 and output tuned circuit L3 – C5. Bias is provided by R1.

One of the key benefits of the cascade circuit is that it effectively minimizes the effect of the Miller capacitance problem associated with single-stage RF amplifiers. The transistors used to implement these amplifiers, BJT, JFET, or MOSFET, all exhibit some form of interelectrode capacitance between collector and base (Ccb) in the BJT and between the drain and gate in FETs (Cdg). This capacitance introduces some feedback that makes it appear that a larger equivalent capacitance, called the Miller capacitance, appears between the base or gate to the ground. This equivalent Miller capacitance Cm is equal to the interelectrode capacitance multiplied by the gain of the amplifier A less 1.

Cm = Cdg(A – 1) or Cm = Ccb(A – 1)

This capacitance forms a low-pass filter with the output impedance of the circuit driving the amplifier. The result is that the upper-frequency limit of the amplifier is limited.

IF Amplifiers | RF Input Amplifiers | Squelch Circuits | Controlling Gain

The cascade circuit of Fig. 9-29 effectively eliminates this problem because the output signal at the drain of Q2 cannot get back to the gate of Q1 to introduce the Miller capacitance. As a result, the cascade amplifier has a much wider upper-frequency range.

Many RF amplifiers become unstable especially at VHF, UHF, and microwave frequencies because of positive feedback that occurs in the interelectrode capacitances of the transistors. This feedback can cause oscillation. To eliminate this problem, some kind of neutralization is normally used, as in RF power amplifiers. In Fig. 9-29, some of the output is feedback through neutralization capacitor C4. This negative feedback cancels the positive feedback and provides the necessary stability.

Although this circuit is shown with JFETs, it can also be built with BJTs or MOSFETs. This circuit is popular in integrated-circuit cell phones and other wireless receivers and is implemented with silicon CMOS, BiCMOS, or SiGe bipolar transistors.

IF Amplifiers

As stated previously, most of the gain and selectivity in a superheterodyne receiver is obtained in the IF amplifier and choosing the right IF is critical to good design. However, today most receivers are of the direct conversion type with I and Q mixers, so no IF stages are used. However, some conventional superheterodyne receivers are still used.

Traditional IF Amplifier Circuits

IF amplifiers, like RF amplifiers, are tuned class A amplifiers capable of providing again in the 10- to the 30-dB range. Usually, two or more IF amplifiers are used to provide adequate overall receiver gain. Fig. 9-30 shows a two-stage IF amplifier. The amplifiers may be single-stage BJT, JFET, or MOSFET transistors or a differential amplifier. Most IF amplifiers are integrated-circuit differential amplifiers, usually either bipolar or MOSFETs.

Older communications receivers used transformer-coupled resonant LC tuned circuits as the filters. However, they cannot provide the superior selectivity needed in today’s wireless applications. Such filters are not used in new designs. Instead, IF amplifiers use crystal, ceramic, or SAW filters for selectivity. They are typically smaller than LC tuned circuits, provide higher selectivity, and require no tuning or adjustment. Today, high-performance communication receivers also use DSP filters to routinely achieve selectivity.

Limiters

In FM receivers, one or more of the IF amplifier stages are used as a limiter, to remove any amplitude variations on the FM signal before the signal is applied to the demodulator. Typically, limiters are simply conventional class A IF amplifi ers. In fact, any amplifi er will act as a limiter if the input signal is high enough. When a very large input signal is applied to a single transistor stage, the transistor is alternately driven between saturation and cutoff.

Driving the transistor between saturation and cutoff effectively flattens or clips of the positive and negative peaks of the input signal, removing any amplitude variations. The output signal is, therefore, a square wave. The most critical part of the limiter design is to set the initial base bias level to that point at which symmetric clipping—i.e., equal amounts of clipping on the positive and negative peaks—will occur. Differential amplifiers are preferred for limiters because they produce the most symmetric clipping. The square wave at the output, which is made up of many undesirable harmonics, is effectively filtered back into a sine wave by the output filter.

IF Amplifiers | RF Input Amplifiers | Squelch Circuits | Controlling Gain

Automatic Gain Control Circuits

The overall gain of a communication receiver is usually selected on the basis of the weakest signal to be received. In most modern communication receivers, the voltage gain between the antenna and the demodulator is in excess of 100 dB. The RF amplifier usually has a gain in the 5- to 15-dB range. The mixer gain is in the 2- to 10-dB range, although diode mixers if used, introduce a loss of several decibels. IF amplifiers have individual stage gains of 20 to 30 dB. Detectors of the passive diode type may introduce a loss, typically from -2 to -5 dB. The gain of the audio amplifier stage is in the 20- to the 40-dB range. Assume, e.g., a circuit with the following gains:

RF amplifier 10 dB
Mixer 22 dB
IF amplifiers (three stages) 27 dB (3 x 27 = 81 total)
Demodulator -3 dB
Audio amplifier -2 dB

The total gain is simply the algebraic sum of the individual stage gains, or 10- 2+27 + 27 + 27 – 3 + 32 = 118 dB.

In many cases, the gain is far greater than that required for adequate reception. Excessive gain usually causes the received signal to be distorted and the transmitted information to be less intelligible. One solution to this problem is to provide gain controls in the receiver. For example, a potentiometer can be connected at some point in an RF or IF amplifier stage to control the RF gain manually. In addition, all receivers include volume control in the audio circuit.

The gain controls cited above are used, in part, so that the overall receiver gain does not interfere with the receiver’s ability to handle large signals. A more effective way of dealing with large signals, however, is to include AGC circuits. As discussed earlier, the use of AGC gives the receiver a very wide dynamic range, which is the ratio of the largest signal that can be handled to the lowest expressed in decibels. The dynamic range of a typical communication receiver with AGC is usually in the 60- to 100-dB range.

Controlling Circuit Gain

If the IF and RF amplifiers are simple common-emitter amplifiers as used in older receivers, AGC can be implemented by controlling the collector current of the transistors. The gain of a bipolar transistor amplifier is proportional to the amount of collector current flowing. Increasing the collector current from some very low level causes the gain to increase proportionately. At some point, the gain flattens over a narrow collector current range and then begins to decrease as the current increases further. Fig. 9-31 shows an approximation of the relationship between the gain variation and the collector current of a typical bipolar transistor. The gain peaks at 30 dB over the 6- to 15-mA range

The amount of collector current in the transistor is, of course, a function of the base bias applied. A small amount of base current produces a small amount of collector current and vice versa. In IF amplifiers, the bias level is not usually fixed by a voltage divider, but controlled by the AGC circuit. In some circuits, a combination of fixed voltage divider bias plus a dc input from the AGC circuit controls overall gain.

  1. The gain can be decreased by decreasing the collector current. An AGC circuit that decreases the current flowing in the amplifier to decrease the gain is called reverse AGC.
  2. The gain can be reduced by increasing the collector current. As the signal gets stronger, the AGC voltage increases; this increases the base current and, in turn, increases the collector current, reducing the gain. This method of gain control is known as forwarding AGC.
IF Amplifiers | RF Input Amplifiers | Squelch Circuits | Controlling Gain

In general, reverse AGC is more common in communication receivers. Forward AGC typically requires special transistors for optimum operation.

Integrated-circuit differential amplifiers are widely used as IF amplifiers. The gain of a differential amplifier is directly proportional to the amount of emitter current flowing. Because of this, the AGC voltage can be conveniently applied to the constant-current source transistor in a differential amplifier. A typical circuit is shown in Fig. 9-32. The bias on constant-current source Q3 is adjusted by R1, R2, and R3 to provide a fixed level of emitter current IE to differential transistors Q1 and Q2. Normally, the emitter-current value in a constant-gain stage is fixed, and the current divide between Q1 and Q2.

The gain is easy to control by varying the bias on Q3. In the circuit shown, increasing the positive AGC voltage increases the emitter current and increases the gain. Decreasing the AGC voltage decreases the gain. Such a circuit is usually inside an integrated circuit with other related circuits.

Fig. 9-33 shows another way to control the gain of an amplifier. Here Q1 is a dual-gate depletion mode MOSFET connected as a class A amplifi er. It may be the RF amplifi er or an IF amplifier. The dual-gate MOSFET actually implements a cascade circuit arrangement, which is common in RF amplifiers. Normal bias is applied via R1 to the lower gate. Additional bias is derived from the source resistor R2. The input signal is applied to the lower gate through C1.

The signal is amplifi ed and appears at the drain where it is coupled through R2 to the next stage. If this circuit is the RF amplifier used ahead of the mixer, LC tuned circuits are normally used at the input and output to provide some initial selectivity and impedance matching. In IF amplifiers, several stages such as this may be cascaded to provide the gain with the selectivity coming from a single crystal, ceramic, SAW, or mechanical filter at the output of the last stage.

The dc AGC control voltage is applied to the second gate through R3. Capacitor C4 is a filter and decoupling capacitor. Since both gates control the drain current, the AGC the voltage varies the drain current which, in turn, controls the transistor gain.

In most modern receivers, the AGC circuits are simply integrated along with the IF amplifier stages inside an IC. Some of these ICs may also have an integrated mixer or integrated demodulator. In some designs, a special IC called a variable gain amplifi er (VGA) is used. It too may be integrated with other circuits. The AGC is controlled by an input voltage derived from an external circuit. Others incorporate the circuits that develop the AGC control voltage.

Deriving the Control Voltage

The dc voltage used to control the gain is usually derived by rectifying either the IF signal or the recovered information signal after the demodulator.

In many receivers, a special rectifier circuit devoted strictly to deriving the AGC voltage is used. Fig. 9-34 shows a typical circuit of this type. The input, which can be either the recovered modulating signal or the IF signal, is applied to an AGC amplifier. A voltage-doubler rectifier circuit made up of D1, D2, and C1 is used to increase the voltage level high enough for control purposes. The RC filter R1-C2 removes any signal variations and produces a pure dc voltage. In some circuits, further amplification of the dc control voltage is necessary; a simple IC op-amp such as that shown in Fig. 9-40 can be used for this purpose. The connection of the rectifier and any phase inversion in the op-amp will determine the polarity of the AGC voltage, which can be either positive form negative depending upon the types of transistors used in the IF and their bias connections.

Squelch Circuits

Another circuit found in most communications receivers is a squelch circuit. Also called a muting circuit, the squelch is used to keep the receiver audio turned off until an RF signal appears at the receiver input. Most two-way communication consists of short conversations that do not take place continuously. In most cases, the receiver is left on so that if a call is received, it can be heard. When there is no RF signal at the receiver input, the audio output is simply background noise.

With no input signal, the AGC sets the receiver to maximum gain, amplifying the noise to a high level. In AM systems such as CB radios, the noise level is relatively high and can be very annoying. The noise level in FM systems can also be high; in some cases, listeners may turn down the audio volume to avoid listening to the noise and possibly miss the desired signal. Squelch circuits provide a means of keeping the audio amplifier turned off during the time that noise is received in the background and enabling it when an RF signal appears at the input.

Noise-Derived Squelch

Noise-derived squelch circuits, typically used in FM receivers, amplify the high-frequency background noise when no signal is present and use it to keep the audio turned off. When a signal is received, the noise circuit is overridden and the audio amplifier is turned on.

Continuous Tone-Control Squelch System

A more sophisticated form of squelch used in some systems is known as the continuous tone-coded squelch system (CTCSS). This system is activated by a low-frequency tone transmitted along with the audio. The purpose of CTCSS is to provide some communication privacy on a particular channel. Other types of squelch circuits keep the speaker quiet when no input signal is received; however, in communication systems in which a particular frequency channel is extremely busy, it may be desirable to activate the squelch only when the desired signal is received. This is done by having the transmitter send a very low-frequency sine wave, usually in the 60- to 254-Hz range, that is linearly mixed with the audio before being applied to the modulator. The low-frequency tone appears at the output of the demodulator in the receiver. It is not usually heard in the speaker, since the audio response of most communication systems rolls off beginning at about 300 Hz, but can be used to activate the squelch circuit.

Most modern transmitters using this system have a choice of multiple tone frequencies, so that different remote receiver can be addressed or keyed up independently, providing a nearly private communication channel. The 52 most commonly used tone frequencies (given in hertz) are listed here:

60.0 100.0 151.4 192.8
67.0 103.5 156.7 196.6
69.3 107.2 159.8 199.5
71.9 110.9 162.2 203.5
74.4 114.8 165.5 206.5
77.0 118.8 167.9 210.7
79.7 120.0 171.3 218.1
82.5 123.0 173.8 225.7
85.4 127.3 177.3 229.1
88.5 131.8 179.9 133.6
91.5 136.5 183.5 241.8
94.8 141.3 186.2 250.3
97.4 146.2 189.9 254.1

At the receiver, a highly selective bandpass filter tuned to the desired tone selects the tone at the output of the demodulator and applies it to a rectifier and RC filter to generate a dc voltage that operates the squelch circuit.

The desired signal comes along, the low-frequency tone is received and converted to a dc voltage that operates the squelch and turns on the receiver audio.

Digitally controlled squelch systems, known as digital coded squelch (DCS), are available in some modern receivers. These systems transmit a serial binary code along with the audio. There are 106 different codes used. At the receiver, the code is shifted into a shift register and decoded. If the decode AND gate recognize the code, the squelch
the gate is enabled and passes the audio.

SSB and Continuous-Wave Reception

Communication receivers designed for receiving SSB or continuous-wave signals have a built-in oscillator that permits recovery of the transmitted information. This circuit called the beat frequency oscillator (BFO) is usually designed to operate near the IF and is applied to the demodulator along with the IF signal containing the modulation.

Recall that the basic demodulator is a balanced modulator (see Fig. 9-35). A balanced modulator has two inputs, the incoming SSB signal at the intermediate frequency and a carrier that mixes with the incoming signal to produce the sum and difference frequencies, the difference being the original audio. The BFO supplies the carrier signal at the IF to the balanced modulator. The term beat refers to the difference frequency output. The BFO is set to a value above or below the SSB signal frequency by an amount equal to the frequency of the modulating signal. It is usually made variable so that its frequency can be adjusted for optimum reception.

Varying the BFO over a narrow frequency range allows the pitch of the received audio to change from low to high. It is typically adjusted for most natural voice sounds. BFOs are also used in receiving CW code. When dots and dashes are transmitted, the carrier is turned off and on for short and long periods of time. The amplitude of the carrier does not vary, nor does its frequency; however, the on/off nature of the carrier is, in essence, a form of amplitude modulation.

Consider for a moment what would happen if a CW signal were applied to a diode detector or other demodulator. The output of the diode detector would be pulses of dc voltage representing the dots and dashes. When applied to the audio amplifier, the dots and dashes would blank the noise but would not be discernible. To make the dots and dashes audible, the IF signal is mixed with the signal from a BFO.

The BFO signal is usually injected directly into the balanced modulator, as shown in Fig. 9-35, where the CW signal at the IF signal is mixed, or heterodyned, with the BFO signal. Since the BFO is variable, the difference frequency can be adjusted to any desired audio tone, usually in the 400- to the 900-Hz range. Now the dots and dashes appear as an audio tone that is amplified and heard in a speaker or earphones. Of course, the BFO is turned off for standard AM signal reception.

Noise Level & Types | Conversion Receivers | Signal-to-Noise Ratio (SNR) ( IF Amplifiers | RF Input Amplifiers | Squelch Circuits | Controlling Gain )

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Transmitter Fundamentals | Frequency Synthesizers | Digital Transmitters ( IF Amplifiers | RF Input Amplifiers | Squelch Circuits | Controlling Gain )

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Reference : Electronic communication by Louis Frenzel

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