@@ -73,7 +73,8 @@ Optional fields:
 - `color`, `fill`: CSS colors.
 - `reverse`, `flip`: SchemDraw orientation controls.
 
-Common `kind` values include `resistor`, `resistor-iec`, `capacitor`,
+Common `kind` values include `bjt-npn`, `bjt-pnp`, `resistor`,
+`resistor-iec`, `capacitor`,
 `capacitor-polarized`, `inductor`, `diode`, `zener`, `schottky`, `led`,
 `source-v`, `source-i`, `source-pulse`, `source-sin`, `buffer`, and `block`.
 
 
@@ -19,7 +19,7 @@ The repository contains the infrastructure needed for this direction:
 - `circuits/a-devices` demonstrates a strong example format: runnable
   netlists, `.SAVE`, `.PLOT`, `.MEAS`, explanatory guides, and automated
   verification.
-- `circuits/cookbook` contains four complete application-oriented recipes
+- `circuits/cookbook` contains five complete application-oriented recipes
   across the pure-SPICE and CustomComponents tracks.
 - `tools/cookbook-schematic` renders deterministic, accessible SVG schematics
   from a SPICE netlist plus a compact `schematic.toml` layout. Its validator
@@ -42,8 +42,8 @@ equations, assumptions, measurements, and educational value.
 
 The initial cookbook milestone is complete:
 
-- Rectifier power supply, PWM DAC, ideal-diode power OR, and simple PLL recipes
-  are present in the user-facing catalog.
+- Rectifier power supply, PWM DAC, BJT audio preamplifier, ideal-diode power
+  OR, and simple PLL recipes are present in the user-facing catalog.
 - Every current recipe has a runnable `.cir`, full guide, checked-in
   `schematic.toml`, generated `schematic.svg`, and accessible `response.svg`.
 - All current schematic layouts display every parsed netlist component and
@@ -52,9 +52,10 @@ The initial cookbook milestone is complete:
 - The data-driven `CircuitCookbookTests` suite compiles and simulates all
   current recipes and enforces their measurement ranges.
 
-The next circuit should be the BJT audio preamplifier. It expands the
-pure-SPICE track with operating-point and AC analysis instead of adding another
-transient-only example.
+The next circuit should be the RC sensor-noise filter and input protector. It
+adds an approachable Simple recipe after the Medium BJT preamplifier while
+introducing frequency-domain attenuation, step response, and clamp-current
+measurements.
 
 ## Cookbook Tracks
 
@@ -87,7 +88,7 @@ story. These recipes should be approachable without prior simulator expertise.
 | Circuit | Track | Status | Practical use | What it measures and teaches |
 | --- | --- | --- | --- | --- |
 | PWM-to-analog converter | Pure SPICE | Complete | Recover an analog control voltage from a microcontroller PWM output | Duty-cycle average, ripple, settling time, and cascaded filtering |
-| RC sensor-noise filter and input protector | Pure SPICE | Planned | Condition a noisy low-voltage sensor before an ADC input | Cutoff frequency, step response, attenuation, and clamp current |
+| RC sensor-noise filter and input protector | Pure SPICE | Next | Condition a noisy low-voltage sensor before an ADC input | Cutoff frequency, step response, attenuation, and clamp current |
 | BJT relay or solenoid driver with flyback diode | Pure SPICE | Planned | Drive an inductive load safely from a logic-level signal | Base drive, coil current, saturation voltage, turn-off transient, and flyback decay |
 | RC switch debouncer with Schmitt-trigger buffer | CustomComponents | Planned | Convert a bouncing mechanical switch into a clean digital edge | Threshold hysteresis, rejected pulse width, propagation delay, and output edge count |
 | Loaded voltage divider and emitter-follower buffer | Pure SPICE | Planned | Scale a battery or sensor voltage without heavily loading its source | Divider error, input impedance, output impedance, bias error, and headroom |
@@ -104,7 +105,7 @@ should introduce realistic design tradeoffs while remaining easy to modify.
 | --- | --- | --- | --- | --- |
 | Rectifier, reservoir, and Zener regulator | Pure SPICE | Complete | Produce filtered low-voltage DC from an isolated AC secondary | Startup, diode conduction, reservoir ripple, load regulation, and load steps |
 | Ideal-diode redundant power input | CustomComponents | Complete | OR two supplies with automatic failover and reverse-current blocking | Source priority, failover dip, handback, standby current, and reverse current |
-| BJT audio preamplifier | Pure SPICE | Next | Raise a small audio signal to a useful level for a following stage | Bias point, AC gain, bandwidth, clipping, loading, and Fourier distortion |
+| BJT audio preamplifier | Pure SPICE | Complete | Raise a small audio signal to a useful level for a following stage | Bias point, AC gain, bandwidth, clipping, loading, and Fourier distortion |
 | 555 monostable pulse stretcher and PWM controller | Packaged subcircuit | Planned | Create a fixed-duration event pulse and an adjustable LED or fan drive | Pulse width, duty range, frequency, reset behavior, and load response |
 | Active anti-alias filter and ADC buffer | Pure SPICE | Planned | Limit sensor bandwidth and drive a sampling input cleanly | Passband gain, cutoff, attenuation, phase shift, settling, and output loading |
 | Class-AB headphone or line-output buffer | Pure SPICE | Planned | Drive a low-impedance audio load from a small-signal source | Quiescent current, crossover distortion, voltage swing, output power, and efficiency |
@@ -128,8 +129,8 @@ simulations and a deeper stability or timing explanation.
 | Linear bench supply with current limiting and foldback | Pure SPICE | Planned | Provide a regulated DC output that survives overload and short-circuit conditions | Line regulation, load regulation, loop response, current limit, foldback, and device dissipation |
 | Digital frequency counter and tachometer | CustomComponents | Planned | Count input events over a fixed gate interval and retain a readable result | Gate timing, count accuracy, overflow, latch timing, reset sequencing, and low-frequency error |
 
-Keep the backlog balanced as recipes are completed. After the BJT
-preamplifier, prefer a simple circuit before starting another difficult one.
+Keep the backlog balanced as recipes are completed. Build the Simple RC
+sensor-noise filter next before starting another Difficult circuit.
 This roadmap is a candidate pool, not a commitment to publish all 24 circuits.
 Apply the selection criteria before promotion and keep the published catalog
 small enough to maintain well.
@@ -150,8 +151,9 @@ The starting sequence established three complementary examples:
    Demonstrated that CustomComponents can form a complete mixed-signal system,
    rather than only isolated device examples.
 
-The ideal-diode redundant power input was the first follow-on application. The
-next addition is the BJT audio preamplifier.
+The ideal-diode redundant power input was the first follow-on application, and
+the BJT audio preamplifier was the second. The next addition is the Simple RC
+sensor-noise filter and input protector.
 
 ## Current Repository Structure and Growth Path
 
@@ -175,6 +177,12 @@ circuits/
         schematic.toml
         schematic.svg
         response.svg
+      bjt-audio-preamplifier/
+        README.md
+        bjt-audio-preamplifier.cir
+        schematic.toml
+        schematic.svg
+        response.svg
     custom-components/
       ideal-diode-power-or/
         README.md
@@ -274,7 +282,7 @@ Each guide should begin with a compact table similar to this:
 | Requires | SpiceSharpParser core |
 | Dialect | Portable SPICE |
 | Analyses | OP, AC, TRAN |
-| Difficulty | Intermediate |
+| Difficulty | Medium |
 | Verified with | SpiceSharpParser 3.4.x |
 
 For custom circuits, `Requires` should explicitly name
@@ -366,7 +374,8 @@ stable across several additional circuits.
 
 ## Immediate Next Steps
 
-1. Add the BJT audio preamplifier with OP, AC, and transient measurements.
+1. Add the RC sensor-noise filter and input protector with AC, transient, and
+   clamp-current measurements.
 2. Define and trial the recipe manifest on the BJT recipe and one existing
    pure-SPICE recipe.
 3. Migrate the remaining cookbook test cases after the manifest format is
 
@@ -9,10 +9,11 @@ documented design calculations, and automated regression coverage.
 
 | Circuit | Track | Analyses | Difficulty | Main lesson |
 | --- | --- | --- | --- | --- |
-| [Rectifier power supply](cookbook/pure-spice/rectifier-power-supply/README.md) | Pure SPICE | TRAN | Intermediate | Rectification, reservoir ripple, Zener regulation, and a load step |
-| [PWM digital-to-analog converter](cookbook/pure-spice/pwm-dac/README.md) | Pure SPICE | TRAN | Beginner | Duty-cycle averaging, ripple attenuation, and settling |
-| [Ideal-diode redundant power input](cookbook/custom-components/ideal-diode-power-or/README.md) | CustomComponents | TRAN | Intermediate | Supply OR-ing, automatic failover, handback, and reverse-current blocking |
-| [Simple phase-locked loop](cookbook/custom-components/simple-pll/README.md) | CustomComponents | TRAN | Advanced | Phase detection, loop filtering, and voltage-controlled oscillation |
+| [Rectifier power supply](cookbook/pure-spice/rectifier-power-supply/README.md) | Pure SPICE | TRAN | Medium | Rectification, reservoir ripple, Zener regulation, and a load step |
+| [PWM digital-to-analog converter](cookbook/pure-spice/pwm-dac/README.md) | Pure SPICE | TRAN | Simple | Duty-cycle averaging, ripple attenuation, and settling |
+| [BJT audio preamplifier](cookbook/pure-spice/bjt-audio-preamplifier/README.md) | Pure SPICE | OP, AC, TRAN, FOUR | Medium | Biasing, gain, bandwidth, coupling, loading, and harmonic distortion |
+| [Ideal-diode redundant power input](cookbook/custom-components/ideal-diode-power-or/README.md) | CustomComponents | TRAN | Medium | Supply OR-ing, automatic failover, handback, and reverse-current blocking |
+| [Simple phase-locked loop](cookbook/custom-components/simple-pll/README.md) | CustomComponents | TRAN | Difficult | Phase detection, loop filtering, and voltage-controlled oscillation |
 
 ## Other Runnable Examples
 
 
@@ -0,0 +1,206 @@
+# BJT Audio Preamplifier
+
+This circuit is a single-transistor common-emitter voltage amplifier for a
+small audio or sensor signal. A 20 mV peak, 1 kHz input from a 600 ohm source
+becomes approximately 0.678 V peak-to-peak at a 47 kohm load. The stage uses a
+12 V supply and biases its collector near mid-supply for useful output
+headroom.
+
+| Property | Value |
+| --- | --- |
+| Requires | `SpiceSharp-Parser` |
+| Dialect | Portable-style SPICE |
+| Analyses | OP, AC, TRAN, FOUR |
+| Difficulty | Medium |
+| Verified with | SpiceSharpParser 3.4.x repository tests |
+| External cross-check | Not yet performed |
+
+## Design Targets
+
+| Target | Value |
+| --- | ---: |
+| Supply | 12 V |
+| Source impedance | 600 ohm |
+| Load | 47 kohm |
+| Nominal input | 20 mV peak at 1 kHz |
+| Collector bias | About 6 V |
+| Midband voltage gain | 14 to 20 V/V |
+| Audio passband | At least 20 Hz to 20 kHz |
+| Small-signal THD | Below 0.5 percent at 1 kHz |
+
+## Schematic
+
+![Common-emitter BJT audio preamplifier schematic](schematic.svg)
+
+The node and component names match
+[`bjt-audio-preamplifier.cir`](bjt-audio-preamplifier.cir). `RE1` supplies
+local AC feedback, while `RE2` establishes most of the DC emitter
+resistance. `CE` bypasses `RE2` at audio frequencies.
+
+## Runnable Netlist
+
+The complete circuit is in
+[`bjt-audio-preamplifier.cir`](bjt-audio-preamplifier.cir). Its amplifier core
+is:
+
+```spice
+R1 vcc base 82k
+R2 base 0 18k
+Q1 collector base emitter Q2N3904
+RC vcc collector 4.7k
+RE1 emitter emitter_bypass 220
+RE2 emitter_bypass 0 820
+CE emitter_bypass 0 100u
+```
+
+The checked-in transistor model is intentionally compact and reproducible. It
+includes finite current gain, Early effect, junction capacitances, base and
+terminal resistances, and forward/reverse transit times.
+
+## How It Works
+
+`R1` and `R2` form a divider that holds the base near 2.1 V.
+The base-emitter junction subtracts about 0.67 V, leaving the emitter near
+1.44 V. Approximately 1.4 mA then flows through the 1.04 kohm total emitter
+resistance.
+
+The collector current creates a voltage drop across `RC`, placing the collector
+at 5.505 V. When the input voltage rises, collector current rises and the
+collector voltage falls, so a common-emitter stage inverts the signal.
+
+`CIN` prevents the source from disturbing the divider's DC bias. `COUT` removes
+the collector's DC level before the signal reaches `RL`. `RE1` improves
+bias stability, gain predictability, and linearity. `CE` makes `RE2` a low
+AC impedance through most of the audio band, recovering voltage gain without
+giving up its DC feedback.
+
+## Design Calculations
+
+Ignoring base current, the divider predicts:
+
+$$
+V_B=12\frac{18\text{k}}{82\text{k}+18\text{k}}=2.16\ \text{V}
+$$
+
+The simulated base voltage is 2.113 V because the transistor loads the divider.
+With the measured 1.440 V emitter voltage:
+
+$$
+I_E\approx\frac{1.440}{220+820}=1.385\ \text{mA}
+$$
+
+The approximate small-signal emitter resistance is:
+
+$$
+r_e\approx\frac{25.8\ \text{mV}}{1.385\ \text{mA}}=18.6\ \Omega
+$$
+
+At midband, `CE` largely bypasses the 820 ohm resistor. A first-order gain
+estimate is therefore:
+
+$$
+|A_v|\approx\frac{4.7\text{k}\parallel47\text{k}}{220+r_e}
+=17.9\ \text{V/V}
+$$
+
+The measured gain is 16.94 V/V after including the transistor model, source
+loading, and incomplete bypass impedance. From a 20 mV peak input, this predicts
+roughly 0.678 V peak-to-peak, matching the transient measurement.
+
+## Simulation Setup
+
+`.OP` verifies the quiescent base, emitter, and collector voltages. `.AC` uses a
+1 V small-signal source and sweeps 10 Hz to 100 MHz with 40 points per decade.
+This makes `VM(output)` numerically equal to voltage gain in V/V and captures
+both coupling-capacitor and transistor-model roll-off.
+
+`.TRAN` runs 20 cycles of the 1 kHz input with a 2 us maximum timestep.
+Measurements use the final five cycles after coupling-capacitor startup has
+settled. `.FOUR` analyzes the last complete output period through the ninth
+harmonic.
+
+`.SAVE` requests the source, input, base, collector, emitter, and output
+voltages. `.PLOT` requests the AC output magnitude.
+
+![Verified BJT preamplifier response summary](response.svg)
+
+## Verified Measurements
+
+| Measurement | Expected range | Verified result |
+| --- | ---: | ---: |
+| Collector DC bias | 4.5 to 6.5 V | 5.505 V |
+| Base DC bias | 1.9 to 2.3 V | 2.113 V |
+| Emitter DC bias | 1.2 to 1.7 V | 1.440 V |
+| Gain at 20 Hz | 11 to 16 V/V | 13.309 V/V |
+| Gain at 1 kHz | 14 to 20 V/V | 16.941 V/V |
+| Gain at 20 kHz | 14 to 20 V/V | 16.942 V/V |
+| Gain at 1 MHz | 13 to 20 V/V | 16.437 V/V |
+| Lower -3 dB point | 10 to 20 Hz | 16.077 Hz |
+| Upper -3 dB point | 2 to 8 MHz | 4.002 MHz |
+| Settled output swing | 0.55 to 0.80 V p-p | 0.678 V p-p |
+| Settled output average | -10 to 10 mV | 0.884 mV |
+
+The netlist-native Fourier result reports 0.108 percent THD at 1 kHz. The
+second harmonic is 0.108 percent of the fundamental; the remaining measured
+harmonics are much smaller. Fourier output is asserted separately from the
+`.MEAS` table because `.FOUR` produces structured harmonic data rather than a
+scalar `.MEAS` result.
+
+## Experiments
+
+- Remove `CE` to observe the large gain reduction and improved linearity from
+  the full 1.04 kohm emitter degeneration.
+- Reduce `CIN` to 100 nF and measure the higher low-frequency cutoff.
+- Change `RL` to 10 kohm to see collector loading reduce the gain.
+- Raise the input amplitude until the collector approaches saturation or the
+  supply rail, then compare the waveform and THD with the small-signal result.
+- Change `RE1` to trade gain for linearity and bias tolerance.
+- Sweep transistor current gain or temperature to see why divider bias and
+  emitter degeneration matter.
+
+## Model Boundary
+
+This example is a voltage preamplifier, not a power amplifier. The 47 kohm load
+represents a following high-impedance stage; it cannot drive a speaker or low-
+impedance headphones directly.
+
+The compact generic 2N3904-style model is suitable for teaching bias,
+small-signal gain, coupling, and first-order distortion. It does not guarantee
+the behavior of a particular vendor part. The circuit omits resistor and
+capacitor tolerances, transistor noise, microphone bias, PCB coupling,
+electromagnetic interference, thermal self-heating, supply decoupling, and a
+volume control. The simulated upper cutoff depends strongly on the compact
+model's capacitance and transit-time parameters.
+
+## Running and Testing
+
+Compile and simulate it from C#:
+
+```csharp
+using System;
+using SpiceSharpParser;
+using SpiceSharpParser.Common;
+
+SpiceCompilationResult result =
+    SpiceCompiler.CompileFile("bjt-audio-preamplifier.cir");
+
+if (!result.Success)
+{
+    throw new InvalidOperationException(
+        string.Join(Environment.NewLine, result.Diagnostics));
+}
+
+foreach (var simulation in result.Model.Simulations)
+{
+    simulation.Execute(result.Model.Circuit);
+}
+```
+
+Run the cookbook quality gate and regression test:
+
+```powershell
+.\tools\cookbook-schematic\.venv\Scripts\cookbook-report
+
+dotnet test src/SpiceSharpParser.Tests/SpiceSharpParser.Tests.csproj `
+  --filter 'FullyQualifiedName~CircuitCookbookTests'
+```
@@ -0,0 +1,38 @@
+BJT audio preamplifier
+VCC vcc 0 12
+VSIG source 0 SIN(0 20m 1k) AC 1
+RS source input 600
+CIN input base 1u
+R1 vcc base 82k
+R2 base 0 18k
+Q1 collector base emitter Q2N3904
+RC vcc collector 4.7k
+RE1 emitter emitter_bypass 220
+RE2 emitter_bypass 0 820
+CE emitter_bypass 0 100u
+COUT collector output 10u
+RL output 0 47k
+
+.MODEL Q2N3904 NPN(Is=6.734f Bf=416.4 Br=0.7374 Vaf=74.03
++ Cje=3.638p Cjc=4.493p Rb=10 Rc=1 Re=0.1 Tf=301.2p Tr=239.5n)
+
+.OP
+.AC DEC 40 10 100Meg
+.TRAN 2u 20m 0 2u
+
+.MEAS OP collector_bias FIND V(collector) AT=0
+.MEAS OP base_bias FIND V(base) AT=0
+.MEAS OP emitter_bias FIND V(emitter) AT=0
+.MEAS AC gain_20hz FIND VM(output) AT=20
+.MEAS AC gain_1khz FIND VM(output) AT=1k
+.MEAS AC gain_20khz FIND VM(output) AT=20k
+.MEAS AC gain_1mhz FIND VM(output) AT=1Meg
+.MEAS AC lower_3db WHEN VM(output)=11.98 RISE=1
+.MEAS AC upper_3db WHEN VM(output)=11.98 FALL=1
+.MEAS TRAN output_pp PP V(output) FROM=15m TO=20m
+.MEAS TRAN output_average AVG V(output) FROM=15m TO=20m
+
+.FOUR 1k V(output)
+.SAVE V(source) V(input) V(base) V(collector) V(emitter) V(output)
+.PLOT AC VM(output)
+.END