If I put my notes here, I might be able to find them again later!
Voltage Regulators for Line Level Audio. Part I : Zeners
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 this time the voltage source AC is set to zero and the current source AC is set to 1. The output impedance is the signal at Vout converted from dB to plain voltage, divided by 1A.
Zout = 10^([dB voltage]/20)/1 ohms
We can see from the data (RR 23 dB, Zout 23 ohms, frequency independent) that this is a pretty lousy voltage regulator - though exactly how bad will become more clear as we move up to bigger and better circuits. The main problem is that the Zener at 5 mA has an impedance of over 20 ohms, and there is a big resistor, R1, between the load and the nice low impedance power supply.
There is no reason to use this circuit in audio. Its like shooting yourself in the foot for no good reason.
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 this time the voltage source AC is set to zero and the current source AC is set to 1. The output impedance is the signal at Vout converted from dB to plain voltage, divided by 1A.
Zout = 10^([dB voltage]/20)/1 ohms
We can see from the data (RR 23 dB, Zout 23 ohms, frequency independent) that this is a pretty lousy voltage regulator - though exactly how bad will become more clear as we move up to bigger and better circuits. The main problem is that the Zener at 5 mA has an impedance of over 20 ohms, and there is a big resistor, R1, between the load and the nice low impedance power supply.
There is no reason to use this circuit in audio. Its like shooting yourself in the foot for no good reason.
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