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curriculum/Electronic Circuit Analysis
section 7 of 104 min read

7. Real-World Stories: Putting It All Together

7.1 An audio amplifier from input to speaker

  1. Microphone preamp. Low-noise differential pair (often a JFET front-end for high input impedance and low voltage noise), high gain, balanced input. Typical voltage gain 40 dB.
  2. Tone controls and EQ. Op-amp-based active filters, bass/mid/treble adjustments, possibly graphic EQ.
  3. Volume control. A potentiometer, or a digital volume control via VCA (voltage-controlled amplifier).
  4. Driver stage. Voltage-amplifier stage (VAS) to drive the output transistors. Almost always a cascode for bandwidth, possibly differential.
  5. Power output. Class-AB complementary symmetry. NPN-PNP pairs of big BJTs (the venerable 2SC5200/2SA1943) or MOSFETs on a heatsink, often Darlingtons or "Sziklai pairs" for high beta.
  6. Negative feedback wraps the whole chain, typically 60 to 80 dB of loop gain, achieving THD below 0.01% even though the output transistors themselves are 1 to 3% nonlinear.
  7. DC servo. An op-amp integrator that watches the speaker DC offset and trims a correction back into the input stage, keeping the speaker terminal near zero volts even though everything else is direct-coupled.
  8. Output Zobel network. A series RC across the output to suppress the high-frequency resonance between the output transformer (if any) and the speaker cable inductance, preserving phase margin.

7.2 An FM radio receiver chain

  1. Antenna. Picks up everything in the FM band and lots more besides.
  2. RF amplifier. Tuned at 88 to 108 MHz, low-noise, narrow band selectivity. Often a JFET or modern HBT with neutralization.
  3. Mixer. Multiplies incoming RF by a local oscillator (LO, set 10.7 MHz away from the desired station). The mixer output contains the IF (intermediate frequency, 10.7 MHz for FM broadcast).
  4. IF strip. Several stagger-tuned amplifiers around 10.7 MHz. Most of the receiver's gain (50 to 80 dB) lives here.
  5. Limiter. Hard-clips amplitude variations. FM is supposed to be constant amplitude, so anything else is interpreted as noise to be discarded.
  6. FM detector. Recovers the audio from frequency variations. Modern receivers use PLL-based detectors or software-defined demodulation after an ADC.
  7. De-emphasis filter. A 50 µs (Europe) or 75 µs (USA) RC filter that undoes the pre-emphasis applied at the transmitter, restoring flat audio response while improving the high-frequency SNR.
  8. Audio amplifier. Drives the speaker.

Every stage uses something from this chapter: high-frequency transistor models (the RF amp), feedback (the IF strip is feedback-stabilized), tuned amplifiers (stagger-tuning), and class-AB power output.

7.3 A class-C transmitter

  1. Modulator. Adjusts the amplitude (AM) or frequency (FM) of the RF carrier in response to the audio input.
  2. Buffer. Isolates the modulated oscillator from heavy load swings. Class-A, low gain, high reverse isolation.
  3. Class-C power stage. Conducts only at peaks of the carrier. The tank circuit reconstitutes a clean sinusoid from the brief current pulses.
  4. Antenna matching network. Transforms the antenna impedance (often 50 Ω) to match the class-C stage's optimum load impedance (perhaps 5 Ω for high efficiency), ensuring efficient power transfer.

A 1 kW AM broadcast transmitter built this way is 80% efficient, with 200 W of heat to dump and 800 W on the air, all controlled by the audio modulator. The same architecture, scaled up, drove every commercial broadcast station in the 20th century.

7.4 Building a CE amp: a SPICE example

Here is a netlist for a textbook common-emitter amplifier you can simulate in ngspice or LTspice to see everything in this chapter come alive:

plaintext
* Common-emitter amplifier, BJT, single-supply
V1   vcc 0      DC 12
R1   vcc b      DC 47k         ; bias divider top
R2   b   0      DC 10k         ; bias divider bottom
RC   vcc c      DC 4.7k        ; collector resistor
RE   e   0      DC 1k
CE   e   0      100u           ; emitter bypass
Cin  in  b      1u             ; input coupling
Cout c   out    1u             ; output coupling
RL   out 0      DC 10k
Q1   c   b   e  Q2N3904
.MODEL Q2N3904 NPN(BF=300 IS=1e-14 VAF=100 CJC=4p CJE=8p TF=0.4n)
Vsig in  0      AC 1
.AC DEC 20 1 100MEG
.PLOT AC V(out)
.END

Run an AC sweep from 1 Hz to 100 MHz and plot V(out). You will see:

  • A low-frequency rolloff around 1/(2π·R·C) of the coupling caps and emitter-bypass cap.
  • A flat midband region.
  • A high-frequency rolloff dominated by the Miller-multiplied CμC_\mu at the input.

Add a cascode (a second BJT above Q1's collector with its base AC-grounded) and the high-frequency cutoff will jump up by an order of magnitude. Wrap a feedback resistor from output to base and the gain will drop while the bandwidth expands, with the gain-bandwidth product approximately conserved. These are the experiments that turn the equations in this chapter into intuition.