MODULE CIRCUIT 11.0504B OPERATION - DESCRIPTION
The control element is the operational NE5534. This is a very low noise operational, especially designed for
very high quality applications in professional audio equipment, control equipment and telephony channel amplifiers.
The operational is internally compensated for a gain equal to or higher than three. Frequency response can be
optimized with an external compensation capacity, for several applications (unity gain amplifier, capacitive load,
slew-rate, low overshoot, etc...).
Characteristics:
Small-signal bandwidth: 10Mhz
Output drive capability: 600
Ω
10V(rms) at Vs=
"
18V
Input noise voltage: 4nV/
%
Hz
DC voltage gain: 100000
AC voltage gain: 6000 at 10KHz
Power bandwidth: 200KHz
Slew-rate: 13V/µs
Supply voltage range:
"
3 to
"
20V
POWER SUPPLY
The BF871 and BF872 transistors are mounted in a common base
configuration, in a current source structure. The current sources have a
double function: polarizing the gate-source links in the MOSFETs to the limit
of the conduction and moving the voltage variations at the operational output
which are refered to ground to voltage variations refered to high voltage
power supply. The polarization point is calculated so the voltage dropout in
Rc (R112+R111) is the limit voltage of conduction of the MOSFETs (
.
2 to
3V), enough to carry the bias current. If we modulate in AC the base-emitter
voltage, the Ic and VRc will vary proportionally. In our configuration, as the
reference voltage Vref is constant (it is a part of the operational power
supply), we add the operational output voltage to the transistors emitter
through Re (R107-R108).
The Rc value fixes the source output impedance. We do not
recommend to raise it higher than 1K
Ω
because of frequency response and
slew rate reasons. This voltage circuit's gain is, as usual in a common base
configuration with Rc/Re emitter resistor, 0.45.
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