Last time, we'd got to a functional voltage regulator, with a pass transistor and op amp error amplifier but I cheated and used ideal voltages for the reference and op amp power supply.
This time I've sketched out a functional circuit using real parts found in the LTSpice library. I've chosen a rail-to-rail op amp to avoid problems with low voltage references. The LT1009 reference puts out 2.5 V, the op amp gain is 4, for an output of 10 V into 1 kohms.
Two versions of the circuit are included below. Voltageregulator5 has some additional RC filter stages to remove noise from the reference and op amp power supply Voltageregulator5b just takes everything straight from the input voltage. As you can see there's a fairly substantial advantage gained from judicial use of RC filtering.
So that's the end of Term 1. The basics have been covered, however briefly. I encourage you to download the LTSpice files and play...
At the end of Part 3 I promised to introduce feedback, and I will, but what we are really talking about here is the addition of the error amplifier, the heart of all modern series pass regulators. The error amplifier is a non-inverting DC signal amplifier, and its function is simple: amplify and buffer the reference voltage. The twist is that the amplifier output is connected to the base of the pass transistor, while the feedback connection is taken from it’s emitter. The pass element is thus placed inside the feedback loop of the error amplifier, improving the ripple rejection and output impedance of the regulator dramatically.
So here we are, the three building blocks of a voltage regulator are in place: the reference voltage, the error amplifier, and the pass device. In the LTSpice circuit I’ve cheated, deliberately, in order to make the operation easier to follow. Instead of building a practical voltage reference I’ve...
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...
Instead of pulling the output current through R1, we add an npn pass transistor, Q1. The output current now "passes" through the transistor, while the Zener diode still regulates the output voltage by being connected to the transistor base.
The output impedance falls to a few ohms, but the ripple rejection improves only slightly.
This is a useful basic circuit block for audio, but the ripple rejection can easily improved by the addition of a couple of additional components, as we'll see shortly.
Posted 15th January 2014 at 11:34 PM byrjm (RJM Audio Blog)
Updated 16th January 2014 at 04:40 AM byrjm
This is the first of a series, where I will be investigating the output impedance and ripple rejection of various voltage regulator circuits using LTSpice.
Today, for the first "lesson" (I'm teaching myself, as much as anything) we will look at the very simple zener voltage regulator.
The load is 1 kohm, and the Zener breakdown voltage is 12 V. The load current is about 10 mA, and to avoid gross inefficiency we will limit the current flowing through the Zener to about 5 mA, by adjusting R1 accordingly. The input voltage is fixed at 18 V.
To measure the ripple rejection, we perform an AC analysis with the voltage source AC set to 1 and the current source AC set to zero. The ripple rejection is the negative value of the signal at Vout: so -20 dB means 20 dB ripple rejection (1 V ripple at Vin generates 0.1 V ripple at Vout at a given frequency.).
To measure the output impedance, again the AC analysis function is used but...