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Model 456A
3 -1 .
INTRODU C TIO N .
The Model 4S6A cons ists of a probe and an amplifier.
Current to be measured flowing through a wire induces
a current in the probe which is clamped around the
wire. This current is then amplified in the amplifier.
3-2.
P R O BE HEAD.
The probe head acts as a 400: 1 step-down cu rrent
transformer.
That is , I milliampere flowing in the
w ire aroun d which the probe is clamped w ill induce
1 /400th of 1 milliampere into the probe secondary.
The output of the probe drives the amplifier.
A MPLIFIER.
3-3.
The amplifier consists of a common- ba se c ircuit
driving a common-emitter output stage.
trans i s tor is used for impedanc e tra nsformation and
the second transistor is used as a current amplifier.
Referring to the schematic you will see that the ac
s ignal from the probe is fed through coupling capac i
tor C2 to the emitter of Q 1 .
The first transistor Ql is used as an input ampli
fier to match the low impedance of the probe to the
higher impedance of trans istor Q2.
is con nected in a common-ba se configuration for low
input impedance and high output impedanc e .
though the current leaving Q 1 is sligh tl y less than
the current going into Ql, there is a power gain since
the impedance level has been inc reased.
signal tha t goes intoQ 1 comes our as a slightly smaller
current but a t a high er impedance level .
The signal then goes through C R1 , an 8- volt breakdow n
diode that furnishes bias
for
to the base of Q2. Transistor Q2 is a common -emitter
s tage which ampl ifies an d reverses the phase of the
signal .
This stage amplifies the curren t and feeds it
back out-of-phase
the emitter of Q 1 . The output is
to
f ed back t o
the emitter of Q1 through R12.
A la rge amount of current feedback is used to feed
back a signal 180 0 out-of-phase with the s ignal and
in parallel with it .
This parallel curren t feedback
res ults in a bette r frequency response, lower input
impedance. dc bias stability, lower dis tortion, etc.
le�
�
3-1.
Figu re
Equivalent Circuit
00395-1
SECTI ON I I I
THEOR Y OF OPER ATION
The fi rst
Transistor Q 1
Even
Thus the
Q2. The signa l is applied
9
10.
and
RD
The equivalen t circuit can be thought of as shown in
figure 3- 1, where
is the current ga in of the tran
CL
sistor.
The transistors have in ternal resis tances as
s hown .
Emitter resistance r e is s mall, but relevant
to this discus s ion , while the collector resistance r c
is so large that it may be neglected .
Referring to figure 3-2 note that the input current is
applied to r e . While this resistance is s mall the probe
should work into zero res istance for best frequency
response .
So it is des i ra bl e to lower the input re
sistance s till further.
Note that the current applied to r e causes a cu rrent
times a s large to flow out of the transis tor. Since
a
is
less than unity, the current flowing out w ill be
a
slightly l ess tha n the current flowing in. Resistors
R8 a nd r c have res is tance much higher than the Z in
of Q2 so most of the cu rrent is fed into Q2. Transis tor
Q2 amplifies and inverts the s ignal and feeds it back
to r e through R12, 9 and 10.
FEEDBACK
INPUT
Figu re 3-2 .
This current is fed back ou t-of-phase and of an am pli
tude almost enough to cancel out the original s ignal.
Thus any voltage developed
current is
almost
cancelled
voltage caused by the cu rren t from Q2. For con s tant
input current the amount of voltage developed at the
input to a device is proportional to its
Since this vol tage developed is reduced by the mecha
nis m expla ined previously. the impedance is lowered.
This resistance consis ting of R12. 10 and 9 is a pproxi
mately 400 ohms and is used as a load across which
the output voltage is developed. The value of 400 oh ms
is used to step-up v Oltage-to-cu rrent ratio 400 times.
The current in the amplifier has been reduced in the
probe to 1 /400th of th e c u rrent being meas ured. The
output voltage is made num erically 400 times th is
so that
reduced current
will exactly equal the num ber of milliamperes flowing
in the wire being measured .
Section 1l I
Pa ragraphs 3- 1 to 3-3
ABOUT
400[1
*(COMBINATION
OF R9,10 a 1 2)
SO-S-S4
Feedback Circuit
r e by the initial
across
out by
the out-of-phase
input impedance.
millivolts at the output
the
3-1
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