HAM Practical Circuits and Components 4 — Questions and Answers
Question 1: What is the purpose of neutralization in an RF amplifier?
- To improve the amplifier's noise figure
- To cancel the feedback through the transistor's internal capacitance that can cause oscillation (Correct answer)
- To match the input impedance to 50 ohms
- To increase the bandwidth of the amplifier
Correct answer: To cancel the feedback through the transistor's internal capacitance that can cause oscillation
Neutralization feeds a portion of the output signal back in opposite phase to cancel the feedback through Cgd or Cbc, preventing self-oscillation.
Question 2: Which type of amplifier configuration is commonly used in the front end of a receiver to provide gain with minimum added noise?
- Class C power amplifier
- Low-noise amplifier (LNA) using a GaAs FET or similar low-noise device (Correct answer)
- Push-pull class B audio amplifier
- Class D switching amplifier
Correct answer: Low-noise amplifier (LNA) using a GaAs FET or similar low-noise device
LNAs using GaAs FETs or modern low-noise transistors are designed to add minimal thermal noise while providing useful gain at RF frequencies.
Question 3: What is the advantage of a differential amplifier input stage in an operational amplifier?
- It provides very high output current
- It rejects common-mode signals (noise present on both inputs equally) while amplifying the differential signal (Correct answer)
- It eliminates the need for a power supply bypass capacitor
- It provides a higher slew rate than single-ended designs
Correct answer: It rejects common-mode signals (noise present on both inputs equally) while amplifying the differential signal
The differential pair rejects common-mode interference (like power supply noise or EMI) that appears equally on both inputs, amplifying only the difference between them.
Question 4: A voltage doubler rectifier circuit produces approximately what output DC voltage compared to the AC input RMS voltage?
- Equal to the RMS voltage
- Equal to the peak voltage (1.414 × RMS)
- Approximately twice the peak voltage (2.828 × RMS) (Correct answer)
- Equal to the average value (0.637 × peak)
Correct answer: Approximately twice the peak voltage (2.828 × RMS)
A voltage doubler charges two capacitors in series, each to the peak voltage, yielding approximately twice the peak (or about 2.83× RMS) at no load.
Question 5: What is the principal cause of intermodulation distortion (IMD) in an RF amplifier?
- Excessive gain in the IF stage
- Non-linearity in the amplifying device creating sum and difference products of input signals (Correct answer)
- Impedance mismatch at the input port
- Thermal noise from the collector resistor
Correct answer: Non-linearity in the amplifying device creating sum and difference products of input signals
Non-linear transfer characteristics cause two or more input signals to mix, creating unwanted intermodulation products at frequencies like 2f1-f2 that fall within the passband.
Question 6: What does the term 'Q point' or 'quiescent point' refer to in transistor amplifier design?
- The frequency at which the amplifier has maximum gain
- The DC operating point (collector current and voltage) with no AC input signal applied (Correct answer)
- The quality factor of the input matching network
- The point at which the transistor begins to saturate
Correct answer: The DC operating point (collector current and voltage) with no AC input signal applied
The Q point is the steady-state DC bias condition — collector current and collector-emitter voltage — that determines the class of operation and sets up the AC signal swing.
Question 7: What is the function of a diplexer at the output of an HF transmitter?
- To combine two antenna signals into one feedline
- To separate transmitted and received signals on the same antenna port by frequency (Correct answer)
- To provide two separate antenna outputs at different power levels
- To eliminate harmonics by frequency-selective filtering
Correct answer: To separate transmitted and received signals on the same antenna port by frequency
A diplexer uses frequency-selective networks to route transmitted and received signals to/from a shared antenna without significant loss or interference between paths.
HAM Practical Circuits and Components 4
What is the purpose of neutralization in an RF amplifier?