E7 - Practical Circuits

A flip-flop is a bistable circuit. An astable Multivibrator is a circuit that continuously alternates between two unstable states without an external clock. Two output level changes are obtained for every two trigger pulses applied to the input of a "T" flip-flop circuit. A characteristic of a monostable multivibrator is that it switches momentarily to the opposite binary state and then returns, after a set time, to its original state. A flip-flop can divide the frequency of pulse train by 2. The number of flip-flops required to divide a signal frequency by 4 is 2.

A truth table is a list of inputs and corresponding outputs for a digital device. The truth tables for important logic elements are:

  • An AND gate produces a logic "1" at its output only if all inputs are logic "1".
  • A NAND gate produces a logic "0" at its output only when all inputs are logic "1".
  • An OR gate produces a logic "1" at its output if any or all inputs are logic "1".
  • A NOR gate produces a logic "0" at its output if any or all inputs are logic "1".

The name for the type of logic which represents a logic "1" as a high voltage is Positive Logic. The name for the type of logic which represents a logic "0" as a high voltage is Negative Logic.


A Class A common emitter amplifier bias would normally be set approximately half-way between saturation and cutoff.

A Class AB amplifier operates more than 180 degrees but less than 360 degrees.

A Push-pull reduces or eliminates even-order harmonics.

A Class C amplifier provides the highest efficiency. When a Class C rather than a class AB amplifier is used to amplify a single-sideband phone signal the signal may become distorted and occupy excessive bandwidth.

To prevent unwanted oscillations in a power amplifier you should install parasitic suppressors and/or neutralize the stage. A vacuum-tube power amplifier can be neutralized by feeding back an out-of-phase component of the output to the input.

When tuning a vacuum tube RF power amplifier that employs a pi-network output circuit, the tuning capacitor is adjusted for minimum plate current, while the loading capacitor is adjusted for maximum permissible plate current.

The circuit shown in Figure E7-1 is a common emitter amplifier and,

  • R1 and R2 are fixed bias
  • R3 is self bias

In the circuit shown in Figure E7-2,

  • C2 is output coupling
  • R is the emitter load

One way to prevent thermal runaway in a transistor amplifier is to use degenerative emitter feedback.

The effect of intermodulation products in a linear power amplifier is the transmission of spurious signals. Third-order intermodulation distortion products are of particular concern in linear power amplifiers because they are relatively close in frequency to the desired signal.

Low input impedance is a characteristic of a grounded-grid amplifier.

A klystron is a VHF, UHF, or microwave vacuum tube that uses velocity modulation.

A parametric amplifier is a low-noise VHF or UHF amplifier relying on varying reactance for amplification.

A FET is generally best suited for UHF or microwave power amplifier applications.


The common name for a filter network which is equivalent to two L networks back-to-back is the Pi network. One advantage of a Pi matching network over an L matching network is that the Q of Pi networks can be varied depending on the component values chosen. The capacitors and inductors of a low-pass filter Pi-network are arranged so that a capacitor is in parallel with the input, another capacitor is in parallel with the output, and an inductor is in series between the two.

A Pi-L network is a network consisting of two series inductors and two shunt capacitors and is used when matching a vacuum-tube final amplifier to a 50-ohm unbalanced output. The advantage a Pi-L-network has over a Pi-network for impedance matching between the final amplifier of a vacuum-tube type transmitter and an antenna is greater harmonic suppression.

A T-network with series capacitors and a parallel (shunt) inductor transforms impedance and is a high-pass filter.

A network transforms a complex impedance to a resistive impedance by canceling the reactive part of the impedance and transforming the resistive part to the desired value.

A Chebyshev filter has ripple in the passband and a sharp cutoff.

An elliptical filter has extremely sharp cutoff, with one or more infinitely deep notches in the stop band. You would use a notch filter to attenuate an interfering carrier signal while receiving an SSB transmission.

An adaptive filter is a digital signal processing audio filter that might be used to remove unwanted noise from a received SSB signal.

A Hilbert-transform filter is a digital signal processing filter that might be used in generating an SSB signal.

A cavity filter would be the best choice for use in a 2-meter repeater duplexer.

Digital modes are most affected by non-linear phase response in a receiver IF filter.


One characteristic of a linear electronic voltage regulator is that the conduction of a control element is varied to maintain a constant output voltage.

One characteristic of a switching electronic voltage regulator is that the control device's duty cycle is controlled to produce a constant average output voltage.

A Zener diode is typically used as a stable reference voltage in a linear voltage regulator.

A series regulator is a type of linear regulator that makes the most efficient use of the primary power source.

A shunt regulator is a type of linear voltage regulator that places a constant load on the unregulated voltage source.

The circuit shown in Figure E7-3 is a linear voltage regulator. The purposes of the components in the circuit are:

  • Q1 increases the current-handling capability of the regulator
  • C1 filters the supply voltage
  • C2 bypasses hum around D1
  • C3 prevents self-oscillation
  • R1 supplies current to D1
  • R2 provides a constant minimum load for Q1
  • D1 provides a voltage reference

Another purpose of a "bleeder" resistor in a conventional (unregulated) power supply is to improve output voltage regulation.

The purpose of a "step-start" circuit in a high-voltage power supply allows the filter capacitors to charge gradually.

All of the following answers are correct. When several electrolytic filter capacitors are connected in series to increase the operating voltage of a power supply filter circuit, resistors should be connected across each capacitor to:

  • To equalize, as much as possible, the voltage drop across each capacitor
  • To provide a safety bleeder to discharge the capacitors when the supply is off
  • To provide a minimum load current to reduce voltage excursions at light loads

The primary reason that a high-frequency inverter type high-voltage power supply can be both less expensive and lighter in weight than a conventional power supply is that the high frequency inverter design uses much smaller transformers and filter components for an equivalent power output.


A reactance modulator on an oscillator can be used to generate FM-phone emissions. It can also be used to produce PM signals by using an electrically variable inductance or capacitance. A phase modulator varies the tuning of an amplifier tank circuit to produce PM signals.

A pre-emphasis network circuit is added to an FM transmitter to proportionally attenuate the lower audio frequencies. A de-emphasis network is added to an FM receiver to restore attenuated lower audio frequencies. A frequency discriminator is a circuit for detecting FM signals.

One result of the process of mixing two signals is the creation of new signals at the sum and difference frequencies. The principal frequencies that appear at the output of a mixer circuit are the original frequencies, and the sum and difference frequencies.

When an excessive amount of signal energy reaches a mixer circuit spurious mixer products are generated.

The process of detection is the recovery of information from a modulated RF signal. A diode detector functions by rectification and filtering of RF signals.

One way a single-sideband phone signal can be generated is by using a balanced modulator followed by a filter. Another common method of generating a SSB signal using digital signal processing is the phasing or quadrature method. A Product detector is well suited for demodulating SSB signals.

“Direct conversion”, when referring to a software defined receiver, is the process where incoming RF is mixed to “baseband” for analog-to-digital conversion and subsequent processing.


One purpose of a marker generator is to provide a means of calibrating a receiver's frequency settings.

A crystal marker generator is a crystal-controlled oscillator that generates a series of reference signals at known frequency intervals. A crystal oscillator followed by a frequency divider would be a good choice for generating a series of harmonically related receiver calibration signals. The additional circuitry that must be added to a 100-kHz crystal-controlled marker generator so as to provide markers at 50 and 25 kHz is two flip-flops.

A conventional frequency counter determines the frequency of a signal by counting the number of input pulses occurring within a specific period of time. The purpose of a frequency counter is to provide a digital representation of the frequency of a signal. The accuracy of a frequency counter is determined by the accuracy of the time base.

The purpose of a prescaler circuit is to divide a higher frequency signal so a low-frequency counter can display the operating frequency. A prescaler would be used to reduce a signal's frequency by a factor of ten. A decade counter digital IC produces one output pulse for every ten input pulses. A 1 MHz oscillator and a decade counter can be combined to produce a 100 kHz fundamental signal with harmonics at 100 kHz intervals.

Other than by directly counting input pulses, period measurement is used by some frequency counters as an alternate method of determining frequency. An advantage of a period-measuring frequency counter over a direct-count type is that it provides improved resolution of signals within a comparable time period.


An operational amplifier is a high-gain, direct-coupled differential amplifier whose characteristics are determined by components external to the amplifier. The most appropriate use of an op-amp RC active filter is as an audio receiving filter. The gain of a theoretically ideal operational amplifier does not vary with frequency.

The advantage of using an op-amp instead of LC elements in an audio filter is that op-amps exhibit gain rather than insertion loss. The gain and frequency characteristics of an op-amp RC active filter is determined by the values of capacitors and resistors external to the op-amp. The frequency and phase response of a filter can cause ringing in the filter. Unwanted ringing and audio instability be prevented in a multi-section op-amp RC audio filter circuit by restricting both gain and Q.

Polystyrene capacitor types are best suited for use in high-stability opamp RC active filter circuits. Sallen-Key is a type of active op-amp filter circuit.


The steps typically followed when selecting the external components for an opamp RC active filter are "standard capacitor values are chosen first, the resistances are calculated, and resistors of the nearest standard value are used". The voltage gain expected from the circuit in Figure E7-4 is given by,

Voltage gain = RF
R1

  • A voltage gain of 47 can be expected when R1 is 10 ohms and RF is 470 ohms.
  • A voltage gain of 38 can be expected when R1 is 1800 ohms and RF is 68 kilohms.
  • A voltage gain of 14 can be expected when R1 is 3300 ohms and RF is 47 kilohms.
  • An output voltage -2.3 volts can be expected if R1 is 1000 ohms, RF is 10,000 ohms, and 0.23 volts is applied to the input.

The term "op-amp input-offset voltage" is the potential between the amplifier input terminals of the op-amp in a closed-loop condition.

The typical input impedance of an integrated circuit op-amp is very high. The typical output impedance of an integrated circuit op-amp is very low.


Three major oscillator circuits often used in Amateur Radio equipment are Colpitts, Hartley and Pierce. A Colpitts and Hartley is the type of oscillator circuit commonly used in VFOs.

The condition that must exist for a circuit to oscillate is that it must have a positive feedback loop with a gain greater than 1. Positive feedback is supplied by a

  • Hartley oscillator through a tapped coil.
  • Colpitts oscillator through a capacitive divider.
  • Pierce oscillator through a quartz crystal.

A phase-locked loop circuit is an electronic servo loop consisting of a phase detector, a low-pass filter and voltage-controlled oscillator. The capture range of a phase-locked loop circuit is the frequency range over which the circuit can lock.

A phase locked loop synthesizer is the type of frequency synthesizer circuit that uses a stable voltage-controlled oscillator, programmable divider, phase detector, loop filter and a reference frequency source.

A direct digital synthesizer is the type of frequency synthesizer circuit that uses a phase accumulator, lookup table, digital to analog converter and a low-pass anti-alias filter. Phase accumulator circuits are classified as a principal component of a direct digital synthesizer (DDS). The phase locked loop circuit is often used in conjunction with a direct digital synthesizer (DDS) to expand the available tuning range. The information contained in the lookup table of a direct digital frequency synthesizer are the amplitude values that represent a sine-wave output. The major spectral impurity components of direct digital synthesizers are spurs at discrete frequencies.

Frequency synthesis and FM demodulation are functions that can be performed by a phase-locked loop.

A stable reference oscillator is normally used as part of a phase locked loop (PLL) frequency synthesizer because any phase variations in the reference oscillator signal will produce phase noise in the synthesizer output. A phase-locked loop is often used as part of a variable frequency synthesizer for receivers and transmitters because it makes it possible for a VFO to have the same degree of stability as a crystal oscillator.

Broadband noise is a major spectral impurity of phase-locked loop synthesizers.

A magnetron oscillator is a UHF or microwave oscillator consisting of a diode vacuum tube with a specially shaped anode, surrounded by an external magnet.

A Gunn diode oscillator is an oscillator based on the negative resistance properties of properly-doped semiconductors.