Voltage Regulators for Line Level Audio. Part III : The Z-reg - diyAudio
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Voltage Regulators for Line Level Audio. Part III : The Z-reg

Posted 17th January 2014 at 04:35 AM by rjm
Updated 20th January 2014 at 10:21 PM by rjm

Part 3 of a series.

I’m going to have to make a detour to point out what we are doing here is learning how these circuits work, and get a very rough idea of their relative merits. We’re not trying to minimize the output impedance, or maximize the ripple rejection. Three reasons immediately come to mind for why it would be bad practice to try and do that:

1. Any such contest will be easily won by the largest capacitor placed on the regulator output.

2. There are clear limits on these parameters after which further “improvement” is unlikely to serve any useful purpose.

3. There are other considerations such as the output noise of the regulator and the stability under dynamic loads, which are equally if not more important.

Clear? No cookies for the “most bestest” circuit in LTSpice. The great utility of LTSpice is it allows you, the designer, to easily check if you’ve left performance on the table and to adjust the values accordingly. Getting fixated on the numbers it feeds you will only end in tears.

So, back to the simple Zener regulated pass transistor circuit from last time, the basic liability here is the reference voltage - at the top of the Zener - still has a good deal of ripple impressed on it. It is not a clean voltage reference. It can be improved by applying a simple RC filter between the reference and the transistor base. Since the output current is flowing through the pass transistor, the RC filter does not add significantly to the output impedance. The ripple rejection increases to over 70 dB in the frequency range 300-300kHz.

This circuit block is what I call the “Z-reg”, and I’ve used it before in the B-board and Sapphire projects. It is quite satisfactory, matching a 3-terminal IC like the LM317 for RR (see figure attached). The output impedance is higher though. The way to tackle that is by feedback, which we’ll come to in the next lesson.
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