Andy C's Page of Audio-Related Topics

Introduction

Welcome to my web site!  This site covers several topics in audio circuit design.  The first is the design of a phono preamp circuit using active RIAA equalization.  The second is some improvements to the publicly-available SPICE models for the ON Semiconductor® MJL3281A and MJL1302A power transistors.  The third deals with using the freeware programs LTspice and FreePCB together to design printed circuit boards.  Brief descriptions follow below.  To go directly to either article, you can click on the links at the top and bottom of the page, or use the links at the end of each article description below.

1. Phono Preamp Design

This article covers the design of a simple and popular active RIAA phono equalization circuit.  The equations developed can also be used for the design of an inverse RIAA network used for lab testing of phono preamps.  Emphasis is placed on a technique for allowing the circuit to implement the Neumann time constant if desired.  Methods for achieving the best possible equalization accuracy are presented.  The impact of finite op-amp gain-bandwidth product and DC open-loop gain on RIAA equalization errors is quantified.  A SPICE technique for isolating the effects of op-amp-induced errors from the total equalization errors is shown.  A complete derivation of the design equations used is provided in an appendix.  This derivation uses a Foster expansion approach not covered elsewhere in the literature.  To go directly to the article, click here.

2. Improved SPICE Models for MJL3281A and MJL1302A

These bipolar power transistors are very popular as output stage transistors for audio power amplifiers.  The SPICE models currently available from ON Semiconductor® for these devices do not yield simulated performance that matches the datasheet measurements very well for some parameters.  This leads to misleading simulation results in many cases.  At a typical quiescent current one would use for a power amplifier output stage design, the SPICE model for the MJL3281A shows a simulated f_T that's low by a factor of six.  The MJL1302A model shows a simulated β that's low by a factor of two to three, depending on collector current.  The article shows graphs of the discrepancies between simulated and measured data.  New models are developed in the article and can be downloaded here.  Detailed information of the model extraction process is presented.  Graphs of the simulated f_T, β and other parameters for the newly-developed model are shown, demonstrating improved correlation with measured device parameters.  To go directly to the article, click here.

A Philosophical Note On SPICE
The power transistor SPICE article borrows heavily from the book Semiconductor Device Modeling with SPICE by Massobrio and Antognetti.  That text contains a wealth of information about the equations used internally by SPICE to simulate device behavior.  Such information is probably of most use to those writing simulator software or support tools.  But there is also much insight to be gained by the end user in studying some of the SPICE modeling equations..  Much of SPICE bipolar transistor modeling is related directly to device physics.  But in its attempt to provide the most detailed information possible to software developers, Massobrio and Antognetti miss some opportunities to provide insight to the end user.  I found that reading the text gave me a fuller understanding of the Early effect and the variation of f_T with collector current, but not until I had filled in some missing pieces.  In the section of this site that describes how the model parameters were extracted, I have tried to pass on what I've learned about these two subjects to the reader.  Looking at the Early effect from the perspective of the Gummel plots is especially thought provoking.  I have also attempted to fill in some missing information in the study of f_T behavior vs. collector current by first relating the behavior of h_FE vs. frequency to the hybrid-π model parameters.  Then the hybrid-π model parameters are connected to the SPICE BJT model parameters to complete the picture.

3. Using LTspice as a Schematic Capture Tool for FreePCB

The LTspice - FreePCB article shows how to use these two freeware programs together to design printed circuit boards.  Using netlist export in LTspice, a netlist is created and imported into FreePCB.  The default behavior of LTspice when assigning pin numbers to devices in the exported netlist often results in incorrect pin numbering.  This in turn would result in an incorrect circuit board design if nothing were done to correct the problem.  This article introduces the concept of "pin-swapping subcircuits" for SPICE.  These pin-swapping subcircuits can be used together with user-created LTspice symbol files to fix the LTspice pin numbering problem.  Once this is done, LTspice and FreePCB become a powerful and free set of tools for creating PCB designs from a working simulation.  To go directly to the article, click here.