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is used to set the single dominant pole in the amplifier's high frequency
response and to drive the power output stage. Thus, generous feedback can
be applied to the circuit to optimize linearity while still maintaining a
comfortable stability margin. Distortion curves are shown in Fig.4.2.
Distortion
%
'
'// / / / / / , V////,
0,6
-d
~
~
I
~
~
- - :
v
~
High)i
High Z Spec.
-j
/
~
':1'/////Y/////////
/ / / / / /
' / / / / :
Low~
Spec,
~
~
'/////-0.
~
'///////_
Low
--
~
0,5
0.4
0.3
0.2
0,1
0
10Hz
20
50
100
200
500
1kHz
5
10
Frequency
z
71022
Fig.4.2. %Distortion. "Low" and "High" Impedance Modes
The type of feedback from the power amplifier is selected by the OUT-
PUT IMPEDANCE switch. In the position "Low", a fraction of the 2707
output voltage is fed back to the input. This gives voltage source character-
istics (constant output voltage, very low output impedance) to the overall
amplifier. If the OUTPUT IMPEDANCE switch is at "High" the 2707 has
feedback proportional to the current flowing in the load. This gives current
source characteristics (constant output current, high output impedance).
However, at very low frequencies the feedback becomes a combination of
both current and voltage feedback and the output impedance is reduced. At
frequencies greater than 10 Hz an even higher output impedance can be
obtained by inserting a shorting link when using the amplifier in the "High"
mode (see section 3.6).
Graphs of output impedance variation over the frequency range, in
"Low" and "High" impedance modes, are shown in Figs. 4.3 and 4.4.
Excessive signal levels at the input will saturate the amplifier and cause
clipping of the output waveform. This will trigger the clipping detector in
the preamplifier, which lights the yellow CLIPPING lamp on the front
panel.
32
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