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4. DESCRIPTION
4.1. GENERAL
Vibration testing of large specimens or testing at high g levels requires a
high power drive to the exciter. However, unless provision is made to avoid
accidental mis-use of
this
power, considerable damage can be done to the
test object, the shaker head or body, or even the amplifier itself. The block
diagram ( Fig.4.1) of the Power Amplifier Type 2707 shows that as well as
power amplification, the instrument protects against electrical, mechanical
and thermal overstressing. Facilities for waveform monitoring and con-
tinuous metering are also provided. Description of the circuit elements
follows.
4.2. INPUT CIRCUIT
The rear-mounted BNC input sockets are connected via the AMPLIFIER
GAIN control and a FET gate to the preamplifier. The 2707 will accept
either an AC coupled input (at the AC INPUT socket) or a direct coupled
input (at the DC INPUT socket). The DC static centring signal from the
DIRECT CURRENT OUTPUT control is applied to the preamplifier
through a similar FET gate. The two gates are normally held open, but if the
protection circuitry is activated the gates are closed and disconnect the
preamplifier input from
the
static centring and amplifier drive signals. The
gate controlling the static deflection signal is purposely made slow-acting so
that large static offsets in table position are not transmitted to the test
object as fast transients when the equipment is switched on or when the
protection circuitry triggers.
The amplifier input impedance is minimum when the AMPLI Fl ER GAl N
control is set for maximum gain, but is never less than 10 kil.
4.3. PREAMPLIFIER
The preamplifier consists of a balanced differential amplifier for inherent
temperature stability, high common mode rejection and a simply set DC
output level. This is followed by a high gain, large voltage swing stage, which
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