BRUEL & KJAER 2305 - Page 11

error signal as soon as this reaches a certain value. That means that the
signal
to the output amplifier section will have a constant value.
Output Amplifier
Seetion. This section consists of a chopper amplifier and
a push - pull type DC power amplifier. The chopper amplifier is introduced
to prevent zero level drift during the amplification of very small DC
signals.
Its chopper frequency is 5 kHz (kc/s). A balanced modulator is used
as chopper, the working principle of which is illustrated in Fig. 1.4. When the
chopper signal has been amplified in two stages it is rectified in a phase-
sensitive rectifier circuit (i.e. balanced demodulator) and then used to
control the DC power stage. This latter stage operates as a type of single-
ended push-pull amplifier, thus allowing one terminal of the drive coil in
the
electromagnetic recording system to be grounded.
+DC--'---~==r------7~------~~--'
Voltage
v_ O ' +---~-- __
- time
Chopper
Oscillator
~--,- __ AC
Voltage
time
fiO¥I,.8
Fig. 1.4. Sketch illustrating the operation of the electronic chopper in the
DC output amplifier.
Writing Speed. From a special winding on the moving coil in the electro·
dynamic drive system a velocity dependent signal is developed . This signal
is introduced as negative feedback to the input of the output amplifier
section . The feedback signal will completely control the output signa l from
this section as long as the signal derived from the second amplitude limiter
has a constant value, which is the case when the limiter is in opera [ion.
In
this way a constant speed of the writing system is obtained. The speed
and thereby the damping (averaging time) of the writing system can be
adjusted by means of the attenuator marked WRITING SPEED. Refer to the
curves Fig. 1.5 and also to part Appendix in this book .
As mentioned under "Lower Limiting Frequency" the stability of the com·
plete writing system is dependent on the combined settings of the WRITING
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