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How to Choose R2 and Cinf for Av=1500 in LTSpice Amplifier Simulation (Vcc=10V, R1=15k)

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How do I set R2 and Cinf in LTSpice to obtain a gain of 1500, measure gain and distortion correctly, and reduce the amplifier gain without changing R1, R2, VCC, or VCM?

To get Av = 1500, sweep R2 in LTSpice until Vout/Vin is 1500, and make Cinf large enough that its reactance is small compared with R1; one suggested value was 10 µF [#21684490] [#21684493] Measure gain as the ratio Vout/Vin using the AC component only, preferably with peak-to-peak values from the waveform cursors, and take the output before Cinf because that capacitor only passes the AC component [#21684481] [#21684483] [#21684490] For distortion, use the FFT display or the .four directive on the output signal [#21684492] Keep vg small enough to stay in the linear region; one reply noted that a 1 V peak input drives the amplifier into saturation, so a much smaller amplitude is needed to minimize distortion [#21684486] If you need to reduce the gain without changing the existing R1/R2/VCC/VCM values, add feedback: put 1 kΩ in series with Q1 base, connect 1,499 kΩ from Q1 base to the collector/output node, add 100 pF from that node to ground for stability, set R2 = 10 kΩ, VCM = 5 V, and Cinf = 10 µF [#21684493]
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Topic summary

✨ The discussion focuses on selecting appropriate values for resistor R2 and capacitor Cinf in an LTSpice amplifier simulation to achieve a theoretical voltage gain (Av) of 1500, with constraints including Vcc=10V, VCM=1.5V, input sine wave frequency of 1 kHz, and R1=15 kΩ. Guidance includes performing parametric sweeps of R2 and Cinf to map gain variations and using regression to derive a formula for gain prediction. Gain measurement is clarified as the ratio of output to input voltage (Vout/Vin), typically expressed in dB as 20·log(Av), with emphasis on measuring peak-to-peak voltages at appropriate nodes, considering DC offsets and signal referencing relative to ground. To minimize output distortion, input amplitude should be chosen carefully, with suggestions to analyze distortion via FFT or harmonic distortion analysis using the ".four" directive in LTSpice. A method to reduce gain without altering R1, R2, Vcc, or VCM involves adding a feedback loop similar to operational amplifier configurations: setting R2 to 10 kΩ for ~1 mA current sources, adjusting VCM to 5 V, inserting a 1 kΩ resistor in series with the inverting input base, adding a 1.499 MΩ resistor for feedback, and using a 100 pF capacitor to prevent oscillations. Cinf is recommended at 10 µF to ensure low reactance compared to R1. The amplifier is powered between Vcc and ground, not symmetric rails, and output saturation and DC offset effects are noted when increasing input amplitude. Simulation techniques include transient and AC sweep analyses, with cursor measurements for voltage levels to calculate gain accurately.
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FAQ

TL;DR: For Av=1500 at 1 kHz, measure gain as Vout/Vin—“The Gain is the ratio between Vout/Vin.” Use LTspice cursors, watch DC offset, and size Cinf so its reactance is small versus R1. [Elektroda, Anonymous, post #21684481] Why it matters: This helps you set R2 and Cinf, choose a clean input amplitude, and avoid saturation in LTspice while meeting your assignment constraints.

Quick Facts

Where do I probe input and output to compute gain in this LTspice amplifier?

Probe the input at Vg after subtracting its DC offset and the output before Cinf (e.g., node at the collectors, V(n007)). Cinf is the coupling capacitor, so measuring before it gives the amplifier’s raw AC output. Then compute Av = Vout/Vin using the AC components only. [Elektroda, Anonymous, post #21684483]

How do I measure voltage gain Av quickly in transient?

Use two cursors to read peak‑to‑peak values. 1) Get Vout(pp) by zooming the output trace. 2) Get Vin(pp) on the input trace. 3) Compute Av = Vout(pp)/Vin(pp). You can sweep R2 to see how Av trends. This method is fast and repeatable in LTspice. [Elektroda, Anonymous, post #21684490]

How do I read gain in dB during an AC sweep?

Gain in dB is 20·log10(Av). In LTspice AC analysis, plot V(out)/V(in) as a trace and read the magnitude in dB. If plotting separately, export Vout and Vin magnitudes and apply 20·log10(Vout/Vin). Remember: Av is a ratio of voltages, not amplitudes with DC included. [Elektroda, Anonymous, post #21684481]

What amplitude should I choose for Vg to minimize output distortion?

Start small and increase until harmonics rise. Use the FFT or the .four directive on the output node to monitor THD. Pick the largest input amplitude that keeps harmonics acceptably low for your spec. “You can try to do a ‘Harmonic distorsion’ with the ‘.four’ directive.” [Elektroda, Anonymous, post #21684492]

Why does the output clip or turn into a square wave?

A small input-stage asymmetry creates DC offset, pushing the output toward one rail. With about 1 V‑peak input, the stage saturates both ways, yielding a squarewave. The duty cycle depends on the offset’s polarity and magnitude. Check the collectors’ DC level to confirm. [Elektroda, Anonymous, post #21684486]

What is Cinf and how big should it be at 1 kHz?

Cinf is the output coupling capacitor. Size it so its reactance is small relative to R1 to avoid low‑frequency roll‑off. An example design uses Cinf=10 µF with R1=15 kΩ, keeping the high‑pass corner well below 1 kHz. This preserves the intended gain at 1 kHz. [Elektroda, Anonymous, post #21684493]

How do I reduce the amplifier gain without changing R1, R2, Vcc, or VCM?

Add global negative feedback from the output to the inverting input through a resistor divider and an input series resistor. This sets closed‑loop gain while leaving internal bias parts untouched. The thread shows an op‑amp‑style loop you can adapt. [Elektroda, Anonymous, post #21684493]

Which node should I use as the official output for gain measurement?

Measure at the junction of the collectors (before Cinf). That node reflects the amplifier’s AC output. Cinf then AC‑couples this signal to the load by removing its DC component. Using this node avoids the coupling capacitor’s frequency‑dependent effects. [Elektroda, Anonymous, post #21684483]

How do I check if the amplifier is already saturated?

Plot the DC level at the collectors. If it sits near ground or near Vcc, the stage is saturated. Increase input amplitude to about 1 V‑peak and look for a squarewave, which confirms saturation and reveals offset through duty‑cycle skew. [Elektroda, Anonymous, post #21684486]

What does VCM mean here, and why does DC offset matter?

VCM is the common‑mode bias at the differential pair inputs. Input‑stage asymmetry introduces a DC offset, which can pull the output toward a rail and reduce headroom. That offset also changes the squarewave duty cycle when the stage clips. [Elektroda, Anonymous, post #21684486]

Can I use feedback to set Av≈1500? Any stable example values?

Yes. One example uses R2=10 kΩ current sources, VCM=5 V, 1 kΩ series at the inverting input, 1.499 MΩ feedback, 100 pF compensation, and Cinf=10 µF to prevent oscillation and set closed‑loop gain ≈1500. “This prevents oscillation.” [Elektroda, Anonymous, post #21684493]

I’m confused: is Vg the output and Vcc the input?

No. Vg is the signal input, offset by a DC bias. The output is at the collectors before Cinf, which then AC‑couples to the next stage. Always measure gain using the AC portions of input and output, not their raw biased levels. [Elektroda, Anonymous, post #21684483]

What’s a quick 3‑step how‑to for measuring Av in LTspice?

1) Measure Vout peak‑to‑peak with cursors on the output trace. 2) Measure Vin peak‑to‑peak with cursors on the input trace. 3) Compute Av = Vout(pp)/Vin(pp); repeat while sweeping R2 if exploring sensitivity. [Elektroda, Anonymous, post #21684490]

How does changing R2 affect gain during experiments?

Use a parametric sweep on R2 and record the resulting Av. Build a table of R2, Cinf, and gain, then fit a curve. This reveals sensitivity and helps you select a combination that hits your target while staying stable. [Elektroda, Anonymous, post #21684479]

What formula should I memorize for quick checks?

Voltage gain Av = Vout/Vin. In decibels, Gain(dB) = 20·log10(Vout/Vin). “Remember that Log is in base 10.” Keep DC offsets out of the calculation by using AC or peak‑to‑peak values from cursors. [Elektroda, Anonymous, post #21684481]

Single‑supply tip: how is this amplifier powered?

It runs between Vcc and ground, not ± supplies. That’s why the input and internal nodes use DC biasing and why the output needs AC coupling through Cinf. Keep this in mind when setting VCM and reading waveforms. [Elektroda, Anonymous, post #21684487]
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