MAS Marquis, Manual

Page: 10 / 22

K O S T A S M E T A X A S D E S I G N
D e s i g n P h i l o s o p h y
08
In a normal Transistor circuit, the 'phase distortion' is 0.5% as compared to
0.000008% for a normal VALVE circuit .
If we monitor the V+ point of the transistor circuit using an oscilloscope, we
would notice this 0.1 Volts, 1.0 kHz signal. If we were to increase the
frequency to 10,000 Hz and up to 1.0 MegaHertz the speed of dynamic
behaviour of the power supply becomes critical. Using a normal I.C. regulator
would result in the signal at V+ actually increasing in amplitude as the
frequency increases to that at 1.0 MegaHertz the 1.0 Volt sine wave is now
over 1.0 Volt!
To fully understand this interaction between the amplifier an power supply,
it is necessary to understand how a voltage regulated power supply works.
A voltage regulated power supply is essentially a D.C. amplifier (not unlike a
normal power amplifier) which instead of having an audio signal at the input
which is then amplified to become a larger audio signal at the output, has a
fixed D.C. voltage reference at the input which is then amplified and
becomes a larger DC voltage of at the output. The output impedance of the
regulator, not unlike the output impedance (or "Damping Factor') of a power
amplifier is less than one ohm at D.C.
If we use a 2.0 Volt zener diode as our fixed DC voltage reference at the
input of the D.C. amplifier which has a gain of 10, the resulting output
voltage is 20 Volts D.C.
The negative feedback loop of the amplifier which fixes the gain of 10 times
the 2.0 Volt zener reference is very important because it maintains the
output voltage irrespective; of an increase or decrease in the power supply
voltage to the amplifier as long as there is a minimum voltage for the
regulator circuit to operate (for a 12 Volt regulator, the minimum voltage
is 15 Volts).
Figure 6. Output Impedance as a Function of
Output Voltage (MC78XXC, AC, B)
, OUTPUT
IMPEDANCE (mą)
O
Ω
10
5.0
3.0
2.0
1.0
0.5
0.3
0.2
0.1
4.0 8.0 12 16 20 24
V
O
, OUTPUT VOLTAGE (V)
Z
f = 120 Hz
I
O
= 500 mA
C
L
= 0 µ F
Figure 4. Ripple Rejection as a Function of
Frequency (MC78XXC, AC, B)
80
50
RR, RIPPLE REJECTION (dB)
0.1 10
f, FREQUENCY (kHz)
0.01
70
40
30
60
1.0
MC78XXB, C, AC
V
in
= 8.0 V to 18 V
I
O
= 500 mA
f = 120 Hz
T
A
= 25 ° C
Figure 3. Ripple Rejection as a Function of
Output Voltages (MC78XXC, AC, B)
80
70
60
50
40
4.0 6.0 8.0 10 12 14 16 18 20 22 24
V
O
, OUTPUT VOLTAGE (V)
RR, RIPPLE REJECTION (dB)
PART #ą ă V
in
MC7805Că= 10 V
MC7806C= 11 V
MC7808Că= 14 V
MC7812Că= 19 V
MC7815Că= 23 V
MC7818Că= 27 V
MC7824Că= 33 V
f = 120 Hz
I
O
= 20 mA
∆
V
in
= 1.0 V(RMS)
DS009063-46