Which circuit is bistable?
AnswerA flip-flop
A flip-flop has two stable states, which is why it is classified as a bistable circuit.
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E7 · Practical Circuits
Amateur Extra · correct answers only, in question-pool order.
What is the function of a decade counter?
AnswerIt produces one output pulse for every 10 input pulses
A decade counter produces one output pulse for every ten input pulses.
Which of the following can divide the frequency of a pulse train by 2?
AnswerA flip-flop
A flip-flop changes state on successive input pulses, so it can divide a pulse-train frequency by two.
How many flip-flops are required to divide a signal frequency by 16?
Answer4
Each flip-flop can divide by two. Four cascaded stages divide by 2⁴, which is 16.
Which of the following circuits continuously alternates between two states without an external clock signal?
AnswerAstable multivibrator
An astable multivibrator has no stable state and continuously alternates between two states without an external clock.
What is a characteristic of a monostable multivibrator?
AnswerIt switches temporarily to an alternate state for a set time
A monostable multivibrator switches to an alternate state for a set interval, then returns to its stable state.
What logical operation does a NAND gate perform?
AnswerIt produces a 0 at its output only if all inputs are 1
A NAND gate is an AND gate followed by inversion, so only the all-1 input condition produces a 0.
What logical operation does an OR gate perform?
AnswerIt produces a 1 at its output if any input is 1
An OR gate produces a logic 1 whenever at least one input is at logic 1.
What logical operation is performed by a two-input exclusive NOR gate?
AnswerIt produces a 0 at its output if one and only one of its inputs is 1
A two-input exclusive-NOR gate outputs 1 when its inputs match and 0 when exactly one input is 1.
What is a truth table?
AnswerA list of inputs and corresponding outputs for a digital device
A truth table lists the possible digital inputs and the corresponding output state for each combination.
What does “positive logic” mean in reference to logic devices?
AnswerHigh voltage represents a 1, low voltage a 0
In positive logic, the higher voltage level represents logic 1 and the lower level represents logic 0.
For what portion of the signal cycle does each active element in a push-pull, Class AB amplifier conduct?
AnswerMore than 180 degrees but less than 360 degrees
Each active device in a push-pull Class AB amplifier conducts for more than half but less than the full signal cycle.
What is a Class D amplifier?
AnswerAn amplifier that uses switching technology to achieve high efficiency
A Class D amplifier uses switching operation rather than linear conduction to achieve high efficiency.
What circuit is required at the output of an RF switching amplifier?
AnswerA filter to remove harmonic content
Switching produces substantial harmonic content, so an output filter is required to remove it.
What is the operating point of a Class A common emitter amplifier?
AnswerApproximately halfway between saturation and cutoff
A Class A common-emitter stage is biased roughly halfway between cutoff and saturation for symmetrical signal swing.
What can be done to prevent unwanted oscillations in an RF power amplifier?
AnswerInstall parasitic suppressors and/or neutralize the stage
Unwanted feedback can make a power amplifier oscillate; parasitic suppressors and neutralization reduce that tendency.
What is a characteristic of a grounded-grid amplifier?
AnswerLow input impedance
A grounded-grid amplifier presents a relatively low impedance at its input.
Which of the following is the likely result of using a Class C amplifier to amplify a single-sideband phone signal?
AnswerSignal distortion and excessive bandwidth
Class C is nonlinear and does not preserve the SSB amplitude envelope, producing distortion and unwanted spectral products.
Why are switching amplifiers more efficient than linear amplifiers?
AnswerThe switching device is at saturation or cutoff most of the time
A switching device dissipates relatively little power when it spends most of its time either fully on or fully off.
What is characteristic of an emitter follower (or common collector) amplifier?
AnswerInput and output signals in-phase
A common-collector, or emitter-follower, stage does not invert phase, so input and output are in phase.
In Figure E7-1, what is the purpose of R1 and R2?
AnswerVoltage divider bias
In Figure E7-1, R1 and R2 establish the transistor's base bias with a voltage divider.
In Figure E7-1, what is the purpose of R3?
AnswerSelf bias
In Figure E7-1, R3 develops emitter voltage that provides self-bias stabilization.
What type of amplifier circuit is shown in Figure E7-1?
AnswerCommon emitter
The transistor stage shown in Figure E7-1 is a common-emitter amplifier.
How are the capacitors and inductors of a low-pass filter Pi-network arranged between the network’s input and output?
AnswerA capacitor is connected between the input and ground, another capacitor is connected between the output and ground, and an inductor is connected between the input and output
A low-pass Pi network uses a capacitor from input to ground, a series inductor, and another capacitor from output to ground.
What is the frequency response of a T-network with series capacitors and a shunt inductor?
AnswerHigh-pass
Series capacitors block lower frequencies while the shunt inductor completes the high-pass T-network response.
What is the purpose of adding an inductor to a Pi-network to create a Pi-L-network?
AnswerGreater harmonic suppression
Adding the extra inductor to a Pi network creates a Pi-L network with greater harmonic attenuation.
How does an impedance-matching circuit transform a complex impedance to a resistive impedance?
AnswerIt cancels the reactive part of the impedance and changes the resistive part to the desired value
An impedance-matching network cancels the reactive component and transforms the remaining resistance to the desired value.
Which filter type has ripple in the passband and a sharp cutoff?
AnswerA Chebyshev filter
A Chebyshev filter trades passband flatness for a steeper cutoff.
What are the characteristics of an elliptical filter?
AnswerExtremely sharp cutoff with one or more notches in the stop band
An elliptical filter achieves an extremely sharp transition and includes one or more transmission zeros in the stopband.
Which describes a Pi-L network?
AnswerA Pi-network with an additional output series inductor
A Pi-L network is a conventional Pi matching network with an additional series inductor on its output.
Which of the following is most frequently used as a band-pass or notch filter in VHF and UHF transceivers?
AnswerA helical filter
Helical resonator filters are commonly used for compact, high-Q band-pass or notch filtering at VHF and UHF.
What is a crystal lattice filter?
AnswerA filter for low-level signals made using quartz crystals
A crystal lattice filter uses quartz resonators to obtain selective filtering of low-level signals.
Which of the following filters is used in a 2-meter band repeater duplexer?
AnswerA cavity filter
Repeater duplexers use high-Q resonant cavities to provide the needed transmit/receive isolation.
Which of the following measures a filter’s ability to reject signals in adjacent channels?
AnswerShape factor
Shape factor compares filter bandwidth at different attenuation levels and indicates how sharply the skirts reject nearby signals.
How does a linear electronic voltage regulator work?
AnswerThe conduction of a control element is varied to maintain a constant output voltage
A linear regulator continuously varies a control element's conduction to hold output voltage constant.
How does a switchmode voltage regulator work?
AnswerBy varying the duty cycle of pulses input to a filter
A switchmode regulator changes the duty cycle of pulses that are then filtered into the desired DC output.
What device is used as a stable voltage reference?
AnswerA Zener diode
A Zener diode operated in breakdown provides a relatively stable reference voltage.
Which of the following describes a three-terminal voltage regulator?
AnswerA series regulator
The usual three-terminal regulator controls a series element between the source and load.
Which of the following types of linear voltage regulator operates by loading the unregulated voltage source?
AnswerA shunt regulator
A shunt regulator controls output by drawing varying current in parallel with the load.
What is the purpose of Q1 in the circuit shown in Figure E7-2?
AnswerIt controls the current to keep the output voltage constant
In Figure E7-2, Q1 acts as the control element whose current is varied to regulate the output voltage.
What is the purpose of C2 in the circuit shown in Figure E7-2?
AnswerIt bypasses rectifier output ripple around D1
In Figure E7-2, C2 bypasses rectifier ripple around the Zener reference D1.
What type of circuit is shown in Figure E7-2?
AnswerLinear voltage regulator
The circuit in Figure E7-2 is a linear regulator using a continuously controlled transistor.
How is battery operating time calculated?
AnswerCapacity in amp-hours divided by average current
Estimated battery operating time is capacity in amp-hours divided by average current in amperes.
Why is a switching type power supply less expensive and lighter than an equivalent linear power supply?
AnswerThe high frequency inverter design uses much smaller transformers and filter components for an equivalent power output
A switchmode supply uses a much higher internal frequency, allowing smaller transformers and filter components for the same power.
What is the purpose of an inverter connected to a solar panel output?
AnswerConvert the panel’s output from DC to AC
The inverter changes the solar array's DC output into AC.
What is the dropout voltage of a linear voltage regulator?
AnswerMinimum input-to-output voltage required to maintain regulation
A linear regulator needs a minimum voltage difference between input and output to remain in regulation.
Which of the following calculates power dissipated by a series linear voltage regulator?
AnswerVoltage difference from input to output multiplied by output current
Power dissipated in the series pass element is approximately the input-output voltage difference multiplied by load current.
Why are equal-value resistors connected across series power-supply filter capacitors to equalize voltage?
AnswerThey equalize the voltage across each capacitor.
The official E7D14 source identifies voltage equalization as one purpose of the equal-value resistors.
How do equal-value resistors across series power-supply filter capacitors improve safety after power is removed?
AnswerThey discharge the capacitors when voltage is removed.
The official E7D14 source identifies discharge after shutdown as one purpose of the resistors.
What load function do equal-value resistors across series power-supply filter capacitors provide?
AnswerThey provide a minimum load on the supply.
The official E7D14 source identifies a minimum-load function as another purpose of the resistors.
What is the purpose of a step-start circuit in a high-voltage power supply?
AnswerTo allow the filter capacitors to charge gradually
A step-start circuit limits initial inrush so the filter capacitors charge more gradually.
Which of the following can be used to generate FM phone signals?
AnswerReactance modulation of a local oscillator
Changing the oscillator's reactance changes its instantaneous frequency and can generate FM phone.
What is the function of a reactance modulator?
AnswerProduce PM or FM signals by varying a capacitance
A reactance modulator electronically varies capacitance, changing oscillator phase or frequency.
What is a frequency discriminator?
AnswerA circuit for detecting FM signals
A frequency discriminator converts frequency variations in an FM signal into the recovered modulation.
What is one way to produce a single-sideband phone signal?
AnswerUse a balanced modulator followed by a filter
A balanced modulator creates a double-sideband suppressed-carrier signal, and a filter removes the unwanted sideband.
What is added to an FM speech channel to boost the higher audio frequencies?
AnswerA pre-emphasis network
Pre-emphasis boosts higher audio frequencies before FM transmission.
Why is de-emphasis used in FM communications receivers?
AnswerFor compatibility with transmitters using phase modulation
The receiver de-emphasis characteristic restores the intended audio response when used with transmitters employing the corresponding emphasis behavior.
What is meant by the term “baseband” in radio communications?
AnswerThe frequency range occupied by a message signal prior to modulation
Baseband is the original information-bearing frequency range before it is translated onto an RF carrier.
What are the principal frequencies that appear at the output of a mixer?
AnswerThe two input frequencies along with their sum and difference frequencies
A mixer output contains components at the two input frequencies and at their sum and difference frequencies.
What occurs when the input signal levels to a mixer are too high?
AnswerSpurious mixer products are generated
Excessive input levels drive a mixer into stronger nonlinearity and create unwanted mixing products.
How does a diode envelope detector function?
AnswerBy rectification and filtering of RF signals
A diode envelope detector rectifies the AM waveform and filters out RF to recover the modulation envelope.
Which type of detector is used for demodulating SSB signals?
AnswerProduct detector
A product detector mixes the SSB signal with a locally generated carrier to recover the audio.
What is meant by “direct sampling” in software defined radios?
AnswerIncoming RF is digitized by an analog-to-digital converter without being mixed with a local oscillator signal
In direct sampling, the ADC digitizes the incoming RF directly rather than first converting it with an analog local-oscillator mixer.
What kind of digital signal processing audio filter is used to remove unwanted noise from a received SSB signal?
AnswerAn adaptive filter
An adaptive filter changes its response as conditions change and can reduce unwanted noise in received SSB audio.
What type of digital signal processing filter is used to generate an SSB signal?
AnswerA Hilbert-transform filter
A Hilbert transform provides the quadrature phase relationship needed for phasing-method SSB generation.
Which method generates an SSB signal using digital signal processing?
AnswerSignals are combined in quadrature phase relationship
Digital SSB generation can combine signal components that are 90 degrees apart to cancel one sideband.
How frequently must an analog signal be sampled to be accurately reproduced?
AnswerAt least twice the rate of the highest frequency component of the signal
Accurate reconstruction requires a sample rate at least twice the highest frequency component in the signal.
What is the minimum number of bits required to sample a signal with a range of 1 volt at a resolution of 1 millivolt?
Answer10 bits
A 1-volt range with 1-millivolt steps requires at least 1000 levels; 10 bits provides 1024 levels.
What function is performed by a Fast Fourier Transform?
AnswerConverting signals from the time domain to the frequency domain
A Fast Fourier Transform reveals the frequency components contained in time-domain samples.
What is the function of decimation?
AnswerReducing the effective sample rate by removing samples
Decimation lowers the effective sampling rate by retaining only selected samples after appropriate filtering.
Why is an anti-aliasing filter required in a decimator?
AnswerIt removes high-frequency signal components that would otherwise be reproduced as lower frequency components
Before decimation, high-frequency content must be removed so it does not alias into lower frequencies.
What aspect of receiver analog-to-digital conversion determines the maximum receive bandwidth of a direct-sampling software defined radio (SDR)?
AnswerSample rate
The ADC sample rate sets the maximum span of RF spectrum that can be represented.
What sets the minimum detectable signal level for a direct-sampling software defined receiver in the absence of atmospheric or thermal noise?
AnswerReference voltage level and sample width in bits
With external noise absent, quantization range and resolution help set the smallest signal the ADC can distinguish.
Which of the following is generally true of Finite Impulse Response (FIR) filters?
AnswerFIR filters can delay all frequency components of the signal by the same amount
A finite-impulse-response filter can be designed for linear phase so all frequency components experience the same delay.
What is the function of taps in a digital signal processing filter?
AnswerProvide incremental signal delays for filter algorithms
Filter taps are delayed signal samples used by the filter algorithm to form the output.
Which of the following would allow a digital signal processing filter to create a sharper filter response?
AnswerMore taps
Increasing the number of taps gives an FIR filter more degrees of freedom for a steeper transition.
What is the typical output impedance of an op-amp?
AnswerVery low
An idealized op-amp is designed to behave like a low-impedance voltage source at its output.
What is the frequency response of the circuit in E7-3 if a capacitor is added across the feedback resistor?
AnswerLow-pass filter
At higher frequencies the capacitor lowers the feedback impedance, reducing closed-loop gain and creating a low-pass response.
What is the typical input impedance of an op-amp?
AnswerVery high
A typical op-amp draws very little input current, so its input impedance is very high.
What is meant by the term “op-amp input offset voltage”?
AnswerThe differential input voltage needed to bring the open loop output voltage to zero
Input offset voltage is the small differential input voltage required to force the open-loop output to zero.
How can unwanted ringing and audio instability be prevented in an op-amp audio filter?
AnswerRestrict both gain and Q
Excessive closed-loop gain and Q can make an active filter ring or become unstable.
What is the gain-bandwidth of an operational amplifier?
AnswerThe frequency at which the open-loop gain of the amplifier equals one
The gain-bandwidth frequency is the unity-gain point of the op-amp's open-loop response.
What voltage gain can be expected from the circuit in Figure E7-3 when R1 is 10 ohms and RF is 470 ohms?
Answer47
For the inverting amplifier in Figure E7-3, absolute voltage gain is RF/R1, so 470/10 = 47.
How does the gain of an ideal operational amplifier vary with frequency?
AnswerIt does not vary with frequency
In the ideal op-amp model, open-loop gain is treated as frequency independent.
What will be the output voltage of the circuit shown in Figure E7-3 if R1 is 1,000 ohms, RF is 10,000 ohms, and 0.23 volts DC is applied to the input?
Answer-2.3 volts
The inverting gain is -RF/R1 = -10, so a +0.23-volt input produces -2.3 volts.
What absolute voltage gain can be expected from the circuit in Figure E7-3 when R1 is 1,800 ohms and RF is 68 kilohms?
Answer38
The absolute inverting gain is RF/R1, giving about 37.8, rounded to 38.
What absolute voltage gain can be expected from the circuit in Figure E7-3 when R1 is 3,300 ohms and RF is 47 kilohms?
Answer14
The absolute inverting gain is RF/R1, giving about 14.
What is an operational amplifier?
AnswerA high-gain, direct-coupled differential amplifier with very high input impedance and very low output impedance
An operational amplifier combines very high input impedance, very low output impedance, and high differential gain.
What are three common oscillator circuits?
AnswerColpitts, Hartley, and Pierce
Colpitts, Hartley, and Pierce are three standard oscillator circuit families.
What is a microphonic?
AnswerChanges in oscillator frequency caused by mechanical vibration
Mechanical vibration can change reactive values or crystal behavior and modulate an oscillator's frequency.
What is a phase-locked loop?
AnswerAn electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator
A phase-locked loop compares phase with a reference and adjusts a voltage-controlled oscillator through a filtered error signal.
How is positive feedback supplied in a Colpitts oscillator?
AnswerThrough a capacitive divider
A Colpitts oscillator obtains positive feedback from two capacitors forming a divider.
How is positive feedback supplied in a Pierce oscillator?
AnswerThrough a quartz crystal
The quartz crystal provides the frequency-selective feedback path in a Pierce oscillator.
Which of these functions can be performed by a phase-locked loop?
AnswerFrequency synthesis and FM demodulation
Phase-locked loops can lock generated frequencies to a reference and can also track FM deviations for demodulation.
How can an oscillator’s microphonic responses be reduced?
AnswerMechanically isolate the oscillator circuitry from its enclosure
Mechanical isolation keeps vibration from changing oscillator frequency.
Which of the following components can be used to reduce thermal drift in crystal oscillators?
AnswerNP0 capacitors
NP0/C0G capacitors have very low temperature coefficient and help stabilize crystal-oscillator frequency.
What type of frequency synthesizer circuit uses a phase accumulator, lookup table, digital-to-analog converter, and a low-pass anti-alias filter?
AnswerA direct digital synthesizer
A direct digital synthesizer numerically advances phase, looks up waveform amplitude, converts it to analog, and filters the output.
What information is contained in the lookup table of a direct digital synthesizer (DDS)?
AnswerAmplitude values that represent the desired waveform
The lookup table converts accumulated phase into amplitude samples for the desired waveform.
What are the major spectral impurity components of direct digital synthesizers?
AnswerSpurious signals at discrete frequencies
Quantization and digital synthesis effects commonly produce spurs at specific frequencies.
Which of the following ensures that a crystal oscillator operates on the frequency specified by the crystal manufacturer?
AnswerProvide the crystal with a specified parallel capacitance
A crystal manufacturer specifies a load capacitance; providing that parallel capacitance makes the oscillator run at the intended frequency.
How can GPS be used to provide a highly accurate oscillator reference for microwave work?
AnswerUse a GPS signal reference.
The official E7H13 source lists a GPS signal reference as one technique for obtaining high oscillator accuracy and stability.
What atomic-frequency reference can provide a highly accurate and stable oscillator for microwave work?
AnswerUse a rubidium stabilized reference oscillator.
The official E7H13 source lists a rubidium-stabilized reference oscillator as a high-accuracy technique.
What resonator technique can provide a highly accurate and stable microwave oscillator?
AnswerUse a temperature-controlled high Q dielectric resonator.
The official E7H13 source lists a temperature-controlled high-Q dielectric resonator as a microwave oscillator-stabilization technique.