Filter
Top Products
22
Figure 5. Operational Amplifier with DC Input Signal
A large electrolytic capacitor is then added across the output terminals of the operational amplifier. The
impedance of this capacitor in the middle range of frequencies (where the overall gain of the amplifier falls off
and becomes less than unity) is much lower than the impedance of any load that might normally be connected to
the amplifier output. Thus, the phase shift through the output terminals is independent of the phase angle of the
load applied and depends only on the impedance of the output capacitor at medium and high frequencies.
Hence, amplifier feedback stability is assured and no oscillation will occur regardless of the type of load
imposed.
In addition, the output stage inside the amplifier block in Figure 4 is removed and shown separately. After these
changes have been carried out, the modified operational amplifier of Figure 5 results.
Replacing the batteries of Figure 5 with rectifiers and a reference zener diode results in the circuit of Figure 6.
A point by point comparison of Figures 3 and 6 reveals that they have identical topology--all connections are
the same, only the position of the components on the diagram differs!
Thus, a series regulated power supply is an operational amplifier. The input signal to this operational amplifier
is the reference voltage. The output signal is regulated dc. The following chart summarizes the corresponding
terms used for an operational amplifier and a power supply.
Operational Amplifier Constant Voltage Power Supply
Input Signal Reference Voltage
Output Signal Regulated DC
Amplifier Regulator
Output Stage Series Regulating Transistor
Bias Power Supply Rectifier/Filter
Gain Control Output Voltage Control
As a result of the specific method used in transforming an operational amplifier into a power supply, some
restrictions are placed on the general behavior of the power supply. The most important of these are:
(1) The large output capacitor Co limits the bandwidth. *