PSpice's four basic analyses: bias point, DC sweep, transient and AC sweep

The PSpice lab's settings and results checked by hand, with the settings that most often trip people up.

The PSpice lab runs each of the four analyses you will use in every later lab once. This guide checks the report's simulation results by hand to show what each analysis computes and where people slip. The schematics are redrawn, and the charts are the same circuits calculated from the formulas.

In this guide 6

1. What the lab is for
2. Bias point: check it by hand
3. DC sweep: a BJT as the supply rises
4. Transient: waveforms in time
5. AC sweep: frequency response
6. Common slips

1. What the lab is for

Analysis	Settings in this lab
Bias Point	defaults, bias voltage and current display on
DC Sweep	`V_VCC` 0 to 6 V, temperature 0, 27, 80 °C
Transient	run to time 3 ms, VSIN 5 V at 1 kHz
AC Sweep	10 Hz to 100 MHz, log, parametric C

Analysis	Where to read the result
Bias Point	numbers on the schematic
DC Sweep	IC(Q1)
Transient	V(R1:1), V(R1:2)
AC Sweep	V(R1:2), DB(V(R1:2))

Bias point: the DC operating point (node voltages and branch currents).
DC sweep: the operating point solved again and again while one DC source steps through a range. A second variable (here, temperature) can be swept on top.
Transient: waveforms against time, the simulator's oscilloscope.
AC sweep: the small-signal frequency response (gain and phase).

2. Bias point: check it by hand

The bias point circuit. A and B are the node names used in the table below.

I = (V1)/(R₁ + (R₂ ‖ R₃) + R₄)loop current

V_A = V1 − I·R₁, V_B = I·R₄node voltages

R2 and R3 in parallel make 10 kΩ, so the loop totals 40 kΩ. Every value PSpice printed on the schematic matches the hand calculation to every digit shown.

Quantity	By hand	PSpice
Top of V1	10 V	10.00 V
Node A	7.5 V	7.500 V
Node B	5 V	5.000 V
Current in R1	250 µA	250.0 µA
Current in R2	125 µA	125.0 µA
Current in R3	125 µA	125.0 µA
Current in R4	250 µA	250.0 µA

Without a ground (named 0) the run stops with a "floating node" error. Every node voltage is measured from ground.
Current display counts current flowing into a part's pin as positive. Mix up the direction and the sign looks wrong.
The suffix M means milli (10⁻³). Write 1 MΩ as 1MEG; 1M is 1 mΩ and gives nonsense.

3. DC sweep: a BJT as the supply rises

The report's DC sweep stepped V_VCC from 0 V to 6 V, plotted the collector current IC(Q1), and overlaid three curves at 0, 27, 80 °C. Two things are worth reading off it.

At low V_CC the curves overlap: below about 0.5 V the transistor is saturated and the current is set by the circuit around it (V_CC and the collector-side resistance), so it follows V_CC almost regardless of temperature.
The flat part is higher when it is hotter: in the active region the current is set by I_B and β. As temperature rises, V_BE falls about 2 mV per °C, so I_B rises, and β grows too. That is why the curves stack in the order 0, 27, 80 °C. The slight upward slope of the flat part is the Early effect.

How far an operating point drifts with temperature, and the fix (an emitter resistor), are worked through in numbers in the β guide and the BJT biasing guide. To overlay temperatures, pick Temperature in the DC sweep's secondary sweep (or a parametric sweep); with nothing set, PSpice simulates at 27 °C.

4. Transient: waveforms in time

The transient and AC sweep circuit. R1 = 100 Ω is worked back from the cutoffs and C values in the report.

f_c = (1)/(2πRC)cutoff

(V_out)/(V_in) = (1)/(√1 + ((f)/(f_c))²), φ = −tan^(⁻¹)((f)/(f_c))amplitude ratio and phase

Δt = (φ)/(360°)·Tphase as a time delay

With a run to time of 3 ms and a 1 kHz input, the plot shows 3 periods. Why input and output looked like one trace in the report follows from the numbers below: the input is only 0.63% of the cutoff frequency.

Quantity	Calculated
Cutoff f_c	159.2 kHz
Frequency ratio f ÷ f_c	0.00628
Amplitude ratio	0.99998
Output amplitude	4.9999 V
Phase	−0.36°
Time delay	1 µs
Periods on screen	3

1 kHz: the two traces overlap

159 kHz (calculated): 0.707 times, 45° behind

The output is only 0.10 mV smaller and 1 µs late, invisible on a 3 ms wide plot. To see the difference, plot V(R1:1) − V(R1:2), or raise the input frequency towards the cutoff.
Leave Maximum Step Size empty and PSpice may take big steps, so a sine looks like a broken line. Set it to a hundredth of a period or less. The time step shown in the report's window was 3 µs, about 333 points per 1 kHz period.
Transient uses VSIN's VAMPL and FREQ and ignores its AC property.

5. AC sweep: frequency response

The same RC circuit was swept logarithmically from 10 Hz to 100 MHz with C stepped through 10 nF, 100 nF, 1 µF (parametric sweep). With the source's AC magnitude at 1 V, the output voltage is the gain, and plotting DB() gives the gain in dB directly.

Gain with R1 = 100 Ω (calculated; same shape as the PSpice plot)

C	f_c	At f_c
10 nF	159 kHz	−3.01 dB
100 nF	15.9 kHz	−3.01 dB
1 µF	1.59 kHz	−3.01 dB

C	At 10 f_c	At 100 MHz
10 nF	−20.0 dB	−56.0 dB
100 nF	−20.0 dB	−76.0 dB
1 µF	−20.0 dB	−96.0 dB

Ten times the C, a tenth of the cutoff: the curves shift sideways by equal steps, one decade each. That is why 1 µF sits furthest left and 10 nF furthest right in the report's plot.
−20 dB per decade above the cutoff: about −20 dB at 10 f_c and −40 dB at 100 f_c. Measuring the slope tells you at once that it is a first-order filter.
Read the −3 dB point with a cursor: on the linear plot find where the output reaches 0.707 V, on the dB plot where it reaches −3 dB, using the Probe cursor, and compare with the formula.
AC sweep treats the circuit as linear, so the output just scales with the input and never clips or distorts. Use a transient run for size and distortion.

Try it: the RC circuit as a transient and an AC sweep

The RC low-pass above (R1, C1, VSIN 5 V) run both ways. It starts with this lab's settings. Unlike real PSpice, points are simply placed one maximum step apart.

Run to Time Maximum Step Size

R1100 Ω C110 nF VSIN frequency1 kHz

Cutoff f_c: 159 kHz
f ÷ f_c: 0.00628
Gain: −0.000171 dB
Phase: −0.36°
Time delay: 1 µs
Periods shown (transient): 3
Points per period (transient): 333

Things to try: empty Maximum Step Size and a run to time of 30 ms turn the sine into a broken line. Raise the frequency near f_c and the output shrinks and lags, and the first few τ show the capacitor starting from 0 V. On the AC sweep, ten times the C1 moves the curve one decade left, and fewer points per decade bend the curve.

6. Common slips

AC sweep shows zero: the source's AC magnitude is empty. With VSIN, enter AC = 1 as well as VAMPL, or use a VAC source.
The parametric sweep does nothing: the part value must be written with braces, {C_var}, and a PARAM part must define C_var with a default.
The transient plot is a broken line: Maximum Step Size is not set.
Run to time is so short that not even one period shows: set it to the number of periods times the period.
A simulation that disagrees with the bench is not automatically wrong. Look at model against real part, and at what the instruments do, separately.

Check cutoffs and time constants with the RC filter calculator, parallel resistance with the series and parallel calculator and a transistor's operating point with the BJT bias calculator. Tables and plots comparing PSpice with bench data can be made with the lab data analyzer.