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Design Notes
APLAC Linear AC and Noise Analysis
This project uses a two-stage differential BJT amplifier design to demonstrate APLAC linear AC and AC noise measurements. Linear AC measurements are limited to voltage, current, and noise, but can be measured anywhere in the circuit. This differs with the wide range of other linear measurements, which are based on port parameters.
Overview
The amplifier is a hierarchical design, where the "Amp_Stage" schematic is referenced twice from the top level schematic, "2_Stg_Amp". The top level schematic also includes the necessary elements to perform the desired measurements, which include Bode plots for one stage and the full amp, a noise contributors list, total output and equivalent input noise, and plots of the contributions of individual devices to the total output noise. Schematic annotations have also been added to show device noise contributions to the output (select the frequency using the tuner), and DC node voltages. After simulation, descend into each stage of the amplifier from the top level to see the annotations associated with it.
"2_Stg_Amp" Schematic
AC voltage and current measurements require an AC source. In this case, an AC_V source is used. The AC_V and AC_I sources include separate ACMag and ACAng parameters for AC measurements, vs. the settings for transient and harmonic balance. ACVS and ACCS sources are always sinusoidal, and use the same magnitude and angle (Mag and Ang) parameters for all measurements: AC, harmonic balance, and transient. These sources can be found in the element browser (click the Elements tab at bottom left of the program window), under Circuit Elements > Sources > AC.
AC Noise measurements additionally require a V_NSMETER element, found in Circuit Elements > MeasDevice. This noise meter identifies the output voltage where the accumulated noise is measured, and the corresponding input source for the equivalent input noise calculation. V_NSMETER can be connected differentially as it is here, or between any node and ground for single-ended outputs.
This schematic uses a custom symbol for the amplifier stages, and uses M_PROBE elements to measure voltages between stages and at the output.
Linear AC Analysis
Like other linear analyses, linear AC analysis begins by calculating the DC operating point of the circuit. All nonlinear elements are modeled by equivalent linear networks at their operating points. AC sources are then applied, each with its own specified magnitude and angle, but all at the same frequency, for each frequency in the specified sweep. The resulting complex-valued AC voltages and currents can then be measured.
Simulation Results
All measurements are performed on the top schematic "2_Stg_Amp". Even the specific device noise contributions are measured in the context of the top level design. All noise measurements are in units of V^2/Hz (Volts squared per Hertz), except for the integrated noise, which is in Volts (RMS).
- DC bias annotation shows the DC voltages at all nodes in the amplifier.
- Noise contribution annotations show the noise contributions of each device to the output noise, as well as the output noise and equivalent input noise at 100MHz.
- "Bode Plot..." graphs show the magnitude and gain of the linear voltage gain, for the first stage and the full amplifier.
- The "Output and Equivalent Input Noise" graph shows exactly that, for the full amplifier.
- "Dev Noise Total" table shows the total noise contribution from each element in the complete amplifier, with the contributions in decreasing order. After simulation, you can use the tuner to choose the frequency for which the list is generated.
- "Integrated Noise" table shows the square root of the integral, over the entire 1GHz sweep, of the output noise and two specific device contributions. The calculations are performed using output equations. Double click on Output Equations in the project browser to see the equations.
- "Specific Device Contributions" graph plots the noise contributions of two specific devices to the total output noise. The device names can be obtained from the "Dev Noise Total" table.