This project presents a simple CMOS PWM Generator designed and simulated in LTspice.
The main goal of the project is to demonstrate the basic operating principle of a pulse-width modulation circuit implemented with CMOS transistor-level blocks.
The circuit generates a PWM output signal by comparing two voltage signals:
- a reference/control voltage
- a periodic ramp signal
The comparator output changes state depending on the relation between these two voltages. As a result, the duty cycle of the output waveform depends on the selected control voltage level.
In this version of the project, the PWM generator is built as a mixed transistor-level and behavioral prototype. The ramp generator is implemented using a behavioral voltage source, while the comparator and output logic are implemented using CMOS transistors.
The implemented circuit operates from a single supply voltage:
VDD = 1.8 V
The behavioral ramp source is configured for an approximate PWM switching frequency of:
f_PWM ≈ 100 kHz
This project can be treated as a first functional prototype of a CMOS PWM generator. It focuses mainly on verifying the basic PWM generation mechanism before moving toward a more complete transistor-level integrated circuit implementation.
Future versions of the project may include:
- transistor-level ramp oscillator
- current mirror biasing
- improved CMOS comparator architecture
- process-corner simulations
- temperature analysis
- layout-oriented design improvements
- full custom IC layout
Pulse-width modulation, commonly abbreviated as PWM, is a technique used to generate a digital switching signal with a controllable duty cycle. Instead of changing the amplitude of the output signal, PWM changes the amount of time for which the signal stays in the high state during one switching period.
The most important parameters of a PWM signal are:
- switching frequency
- period
- duty cycle
- high-level voltage
- low-level voltage
The PWM period is defined as:
T_PWM = 1 / f_PWM
where:
T_PWMis the PWM periodf_PWMis the PWM switching frequency
For this project, the approximate switching frequency is:
f_PWM ≈ 100 kHz
Therefore, the approximate PWM period is:
T_PWM ≈ 10 µs
The duty cycle describes how long the output signal remains high during one period. It can be expressed as:
D = t_ON / T_PWM
where:
Dis the duty cyclet_ONis the time when the PWM signal is in the high stateT_PWMis the full PWM period
The duty cycle is often expressed as a percentage:
D[%] = (t_ON / T_PWM) · 100%
In a basic analog PWM generator, the PWM signal can be created by comparing a control voltage with a periodic ramp signal. The comparator output switches depending on which signal has a higher instantaneous voltage.
The basic operating principle is:
- when the control voltage is higher than the ramp voltage, the comparator output changes to one logic state
- when the ramp voltage becomes higher than the control voltage, the comparator output changes to the opposite logic state
As the control voltage changes, the crossing point between the control voltage and the ramp signal also changes. This directly changes the duty cycle of the output PWM waveform.
In this project, the ramp signal is generated using a behavioral voltage source. This allows the PWM generation mechanism to be tested without designing a complete transistor-level ramp oscillator at this stage.
The simplified PWM generation concept is shown below:
Control voltage ─┐
├── Comparator ── CMOS logic ── PWM output
Ramp signal ─────┘
This approach is useful for early-stage CMOS circuit development because it separates the verification of the comparator and output logic from the design of the ramp generation block.
The CMOS PWM generator is divided into three main functional blocks:
- behavioral ramp generator
- CMOS comparator
- CMOS output logic stage
The simplified block diagram of the implemented circuit is shown below:
Control voltage ─┐
│
├── CMOS Comparator ── CMOS Output Logic ── PWM Output
│
Ramp generator ──┘
The purpose of each block is described in the following subsections.
The ramp generator produces a periodic voltage waveform used as the carrier signal for PWM generation.
In this project, the ramp signal is implemented using a behavioral voltage source in LTspice. This simplifies the first version of the design and allows the comparator and CMOS output logic to be tested without adding the complexity of a transistor-level oscillator.
The ramp signal defines the PWM switching period and therefore determines the approximate PWM frequency:
f_PWM ≈ 100 kHz
For this frequency, the approximate switching period is:
T_PWM ≈ 10 µs
The behavioral ramp generator is not intended to represent the final integrated circuit implementation. It is used only as a practical simulation block for early-stage verification.
The control voltage is the input signal that determines the duty cycle of the PWM output waveform.
Changing the control voltage shifts the crossing point between the control voltage and the ramp signal. As a result, the time during which the comparator output remains in a given logic state also changes.
In general:
- a lower control voltage produces a smaller duty cycle
- a higher control voltage produces a larger duty cycle
This behavior is the basic principle of analog PWM generation.
The comparator is the main decision-making block of the PWM generator. It compares the instantaneous value of the ramp voltage with the selected control voltage.
The comparator output changes its logic state depending on the voltage difference between its two inputs.
The simplified comparison rule is:
If V_CONTROL > V_RAMP → comparator output changes state
If V_RAMP > V_CONTROL → comparator output changes to the opposite state
In this project, the comparator is implemented using CMOS transistors. This makes the design closer to an integrated circuit implementation than a purely behavioral comparator model.
The comparator stage is responsible for converting the analog voltage comparison into a digital-like switching signal.
The output logic stage processes the comparator output and generates the final PWM output waveform.
This block is implemented using CMOS transistors. Its main purpose is to provide a clean switching output compatible with the selected supply voltage:
VDD = 1.8 V
The output waveform switches between the low and high logic levels:
V_LOW ≈ 0 V
V_HIGH ≈ VDD
The final PWM output signal can then be analyzed in the LTspice waveform viewer.
The complete signal flow of the PWM generator can be summarized as follows:
- The behavioral ramp generator produces a periodic ramp waveform.
- The control voltage sets the desired duty cycle.
- The CMOS comparator compares the control voltage with the ramp signal.
- The comparator output changes state at the crossing point between both signals.
- The CMOS output logic generates the final PWM waveform.
This architecture allows the basic PWM generation mechanism to be verified while keeping the first version of the circuit relatively simple and easy to simulate.
The complete LTspice implementation of the CMOS PWM generator is shown below.
The circuit was implemented and simulated in LTspice. The main purpose of this implementation is to verify the functional behavior of a CMOS-based PWM generator before moving toward a more advanced transistor-level integrated circuit design.
The LTspice model consists of both behavioral and transistor-level blocks. The ramp generator is implemented using a behavioral voltage source, while the comparator and output logic are implemented using CMOS transistors.
The main implemented blocks are:
- supply voltage source
- control voltage source
- behavioral ramp voltage source
- CMOS comparator stage
- CMOS output logic stage
- transient simulation setup
The circuit operates from a single supply voltage:
VDD = 1.8 V
This supply voltage defines the high logic level of the CMOS stages and the maximum expected amplitude of the digital PWM output signal.
In the simulation, the supply voltage source provides the required biasing for the CMOS comparator and output logic.
The control voltage is used as one of the comparator input signals. Its value determines the duty cycle of the generated PWM waveform.
In the implemented model, the control voltage is provided by an independent voltage source. By changing this voltage, different PWM duty cycles can be obtained and analyzed.
The general relationship is:
Higher control voltage → higher duty cycle
Lower control voltage → lower duty cycle
This allows the circuit behavior to be tested for different operating points.
The ramp signal is generated using a behavioral voltage source in LTspice. This source creates a periodic ramp waveform that acts as the carrier signal for PWM generation.
The behavioral implementation was selected intentionally for the first version of the project. It allows the PWM principle, comparator behavior, and output logic operation to be verified without designing a full transistor-level ramp oscillator.
The approximate ramp frequency is:
f_PWM ≈ 100 kHz
which corresponds to the switching period:
T_PWM ≈ 10 µs
The ramp signal is compared with the control voltage by the CMOS comparator.
The behavioral ramp source configuration used in LTspice is shown below.
The comparator is implemented using CMOS transistors instead of an ideal behavioral comparator model.
Its task is to compare:
- the control voltage
- the ramp voltage
Depending on the instantaneous voltage difference between these two signals, the comparator output changes its logic state.
This transistor-level implementation makes the design more realistic from the point of view of analog/mixed-signal IC design. It also allows the switching behavior, signal transitions, and output response to be observed directly in simulation.
The output logic stage is also implemented using CMOS transistors. It processes the comparator output and generates the final PWM waveform.
The output signal switches approximately between:
V_LOW ≈ 0 V
V_HIGH ≈ 1.8 V
The purpose of this stage is to provide a digital-like output signal with voltage levels compatible with the selected supply voltage.
The project uses transient analysis to observe the time-domain behavior of the PWM generator.
Transient simulation is required because PWM operation depends on time-varying signals, especially the ramp waveform and the resulting switching output.
The simulation allows the following signals to be observed:
- ramp voltage
- control voltage
- comparator output
- final PWM output
The transient simulation setup used in LTspice is shown below.
The LTspice implementation combines a behavioral ramp source with transistor-level CMOS signal processing blocks. This approach provides a practical first step toward a more complete CMOS PWM generator design.
The current implementation focuses on functional verification of the PWM generation mechanism. More advanced versions may replace the behavioral ramp source with a transistor-level oscillator or current-controlled ramp generator.
The circuit behavior was verified using transient analysis in LTspice.
This type of simulation allows the time-domain operation of the PWM generator to be observed, including the ramp waveform, comparator switching behavior, and final PWM output signal.
The main goal of the simulation was to confirm that the implemented CMOS PWM generator produces a pulse-width modulated output waveform whose duty cycle depends on the selected control voltage.
The simulation was performed to verify the following aspects of the circuit:
- generation of a periodic ramp waveform
- comparison between the ramp signal and the control voltage
- switching behavior of the CMOS comparator
- operation of the CMOS output logic stage
- generation of the final PWM output signal
- relationship between the control voltage and the PWM duty cycle
The most important expected behavior is that changing the control voltage changes the duty cycle of the output waveform.
The PWM generator was analyzed in the time domain using transient simulation.
The approximate PWM switching frequency was set to:
f_PWM ≈ 100 kHz
This corresponds to the approximate PWM period:
T_PWM ≈ 10 µs
The transient simulation time should be long enough to observe several PWM periods. This makes it possible to verify whether the generated waveform is periodic and stable.
An example transient simulation command may be written as:
.tran 0 100u
This simulation time allows approximately ten PWM periods to be observed for a switching frequency of about 100 kHz.
During simulation, the following signals should be observed in the LTspice waveform viewer:
- ramp voltage
- control voltage
- comparator output
- final PWM output
The most important signal is the final PWM output waveform, because it confirms whether the circuit generates the expected switching signal.
The ramp and control voltage signals are used to verify the PWM generation mechanism. The comparator output shows the direct result of the analog comparison before final output processing.
The circuit operates from a single supply voltage:
VDD = 1.8 V
Therefore, the expected logic levels of the final PWM output are approximately:
V_LOW ≈ 0 V
V_HIGH ≈ 1.8 V
In practice, the exact output voltage levels may slightly differ from the ideal values because the comparator and output logic are implemented using CMOS transistors.
To verify PWM duty cycle control, the simulation can be repeated for different control voltage values.
For example, the control voltage may be tested at several levels within the ramp voltage range:
V_CONTROL = 0.3 V
V_CONTROL = 0.6 V
V_CONTROL = 0.9 V
V_CONTROL = 1.2 V
V_CONTROL = 1.5 V
For each control voltage value, the output PWM waveform should have a different duty cycle.
The expected behavior is:
Lower V_CONTROL → lower duty cycle
Higher V_CONTROL → higher duty cycle
This confirms that the duty cycle is controlled by the analog input voltage.
The expected result of the transient simulation is a periodic PWM output waveform switching between low and high logic levels.
The duty cycle of the output waveform should change depending on the selected control voltage. This confirms correct operation of the basic PWM generation mechanism.
The simulation should show that:
- the ramp waveform is periodic
- the control voltage defines the switching threshold
- the comparator changes state when the ramp crosses the control voltage
- the CMOS output logic produces a digital-like PWM waveform
- the output duty cycle depends on the control voltage
The simulation setup verifies the functional operation of the CMOS PWM generator in the time domain. The use of transient analysis is essential because PWM generation is based on periodic time-varying signals.
This simulation stage confirms that the implemented LTspice model can generate a controllable PWM output waveform and provides a basis for further improvements of the circuit.
The transient simulation confirms the basic operation of the CMOS PWM generator.
The circuit generates a periodic PWM output waveform by comparing the control voltage with the ramp signal.
The most important observed signals are:
- ramp voltage
- control voltage
- comparator output
- final PWM output
The simulation results show that the output waveform switches between low and high logic levels and that its duty cycle depends on the selected control voltage.
The ramp waveform acts as the carrier signal for PWM generation.
The control voltage defines the switching threshold for the comparator.
During operation, the comparator continuously compares the instantaneous value of the ramp voltage with the control voltage.
The PWM switching frequency is approximately:
f_PWM ≈ 100 kHz
which corresponds to the switching period:
T_PWM ≈ 10 µs
The relationship between the ramp signal and the control voltage determines the moment when the output changes state.
The simulated ramp and control voltage waveforms are shown below.
The comparator output changes state when the ramp voltage crosses the control voltage level.
This confirms that the CMOS comparator correctly converts the analog voltage comparison into a switching signal.
The comparator output is not an ideal mathematical signal. Its waveform depends on the transistor-level CMOS implementation, including switching delay and transition behavior.
The simulated comparator output waveform is shown below.
The final PWM output is generated by the CMOS output logic stage.
The output signal switches approximately between:
V_LOW ≈ 0 V
V_HIGH ≈ 1.8 V
This confirms that the output logic produces a digital-like waveform compatible with the selected supply voltage.
The simulated PWM output waveform is shown below.
The simulation confirms that the duty cycle of the PWM output depends on the control voltage.
When the control voltage is changed, the crossing point between the control voltage and the ramp signal also changes. This changes the amount of time during which the PWM output remains in the high state.
The expected behavior is:
Lower V_CONTROL → lower duty cycle
Higher V_CONTROL → higher duty cycle
This behavior is the key feature of PWM generation.
Example duty cycle comparison for different control voltage values is shown below.
The output waveform reaches voltage levels close to the selected supply rails.
The expected output levels are:
Low state ≈ 0 V
High state ≈ VDD = 1.8 V
Small deviations from ideal voltage levels may occur because the comparator and output logic are implemented using CMOS transistors rather than ideal behavioral blocks.
This makes the simulation more realistic and useful from the perspective of analog/mixed-signal IC design.
The simulation results confirm that the implemented circuit operates as a functional CMOS PWM generator.
The most important observations are:
- the ramp signal is periodic
- the comparator switches when the ramp voltage crosses the control voltage
- the CMOS output logic generates a digital-like PWM signal
- the output voltage levels are compatible with the
1.8 Vsupply - the duty cycle changes according to the selected control voltage
The obtained results verify the basic PWM generation mechanism and confirm that the project can be used as a foundation for further transistor-level improvements.
The simulation results confirm that the implemented circuit operates as a functional CMOS PWM generator. The output waveform is generated by comparing a periodic ramp signal with a selected control voltage and then processing the comparator output through the CMOS logic stage.
The most important observation is that the PWM output duty cycle changes according to the control voltage level. This confirms the basic operating principle of analog PWM generation.
The PWM signal is generated when the ramp voltage crosses the control voltage level. At this crossing point, the comparator changes its output state.
As a result, the time during which the PWM output remains in the high state depends on the value of the control voltage.
The expected behavior is:
Lower V_CONTROL → shorter high-state time → lower duty cycle
Higher V_CONTROL → longer high-state time → higher duty cycle
This behavior was confirmed in transient simulation.
The final PWM output switches approximately between the supply rails:
V_LOW ≈ 0 V
V_HIGH ≈ 1.8 V
This means that the output logic stage produces a digital-like waveform compatible with the selected CMOS supply voltage.
Small deviations from ideal voltage levels may occur because the comparator and output logic are implemented using CMOS transistors, not ideal behavioral blocks.
The comparator stage is responsible for converting the analog voltage comparison into a switching signal.
Since the comparator is implemented using CMOS transistors, its output behavior is not perfectly ideal. In practical simulation results, the following effects may be observed:
- finite switching time
- propagation delay
- non-ideal transition slopes
- possible sensitivity near the switching threshold
These effects are expected in transistor-level CMOS circuits and make the simulation more realistic than using an ideal voltage-controlled switch or behavioral comparator.
In the current version of the project, the ramp signal is generated using a behavioral voltage source.
This is a simplification, but it is useful at this stage of development because it allows the comparator and output logic to be verified independently from the ramp generator design.
The behavioral ramp source makes it possible to focus on the core PWM mechanism:
Ramp signal + Control voltage → Comparator → CMOS logic → PWM output
However, this also means that the current version is not yet a fully transistor-level IC implementation.
The duty cycle can be estimated from the average value of the PWM output waveform if the output switches approximately between 0 V and 1.8 V.
For a rail-to-rail PWM signal, the relationship is:
Duty cycle [%] ≈ 100 · V_AVG / VDD
In this project, the duty cycle measurement can be performed using LTspice .meas commands, for example:
.meas tran VAVG_BUF AVG V(PWM_BUF) FROM=20u TO=100u
.meas tran DUTY_BUF PARAM 100*VAVG_BUF/1.8
This method is practical for quick duty cycle verification, especially when comparing several control voltage values.
The current design is intentionally simple. It is focused on verifying the PWM generation principle rather than achieving a production-ready integrated circuit.
The main advantages of this approach are:
- simple simulation setup
- clear PWM generation mechanism
- transistor-level CMOS comparator and output logic
- easy duty cycle control using the input voltage
- good starting point for future IC-oriented improvements
The main limitations are:
- behavioral ramp generator
- simplified comparator architecture
- no process-corner analysis
- no temperature analysis
- no layout parasitics
- no full custom IC layout
The obtained simulation results show that the circuit correctly generates a PWM signal whose duty cycle depends on the control voltage. The use of CMOS transistor-level blocks for the comparator and output logic makes the design more relevant from the perspective of analog/mixed-signal IC design.
Although the ramp generator is still behavioral, the project successfully demonstrates the fundamental operation of a CMOS PWM generator and provides a solid base for further development.