BRUEL & KJAER 2706 - 4 Description; Attenuators and Preamplifier

4. DESCRIPTION
A block diagram of the Power Amplifier Type 2706 is shown in Fig.4.1.
Output
Fig.4. 1. Block Diagram of 2706
4.1. ATTENUATORS AND PREAMPLIFIER
The signal is fed to the Power Amplifier via the INPUT socket which
accepts
the standard B & K coaxial plug JP 0101. The input section itself,
including
the preamplifier, is DC coupled, but any DC voltage present at the
input will not appear in the output voltage as the gate following the input
section is AC coupled. The amplifier input impedance is 15 kil.
The input leads to the ATTENUATOR which steps down the signal in
10 dB steps to a maximum attenuation of 40 dB. It is followed by the
GAl N CONTROL potentiometer which allows continuous adjustment of
the voltage down to zero. This is a high resistance logarithmic potentiometer
and has no loading effect on the previous attenuator. This means that the
amplifier input impedance is independent of the ATTENUATOR and GAIN
CONTROL setting. The preamplifier is an operational amplifier with
resistive feedback.
It has a fixed 20 dB voltage gain.
The
signal passes then through a gate which will ground the signal when
excessive
temperature is reached on the heat sink. See Temperature
Protection, section 4.4.
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