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FIGURE 20.47 Bipolar variable gain current-mode amplifier.
Current Control (Mode) Amplifier
As previously discussed, in many electromagnetic actuators, the output force or torque of the device has
strong correlation with the winding current, e.g., for a permanent magnet DC motor and a voice coil
actuator, the output torque and force are proportional to the input current. Therefore, in many motion
control applications, it is more desirable to have a voltage-to-current conversion (current-mode amplifier)
at the power stage, where the input voltage command is proportional to the current flowing into/out of
the motor (winding). Figure 20.43(b) shows a basic circuit for a current-mode amplifier. The relationship
between the emitter (motor) current i M and the input voltage command V IN is
i M = V IN – V BE
----------------------
R S
If the base-emitter voltage is ignored, the voltage across the motor current i M is proportional to the
input voltage V IN , i.e., i M ≈ (1/R S ) · V IN .
Figure 20.47 shows a basic bipolar current-mode amplifier. An Op-Amp is used to close the current
loop. The resistor R S , often called the sensing resistor, is used to sense the motor current for feedback to
the Op-Amp. Depending on the desired current magnitude, the sensing resistor needs to have adequate
2 ⋅
power rating to dissipate the heat (i M R S ) generated by flowing current through the resistor. For a zeroth
order approximation, at steady state, the Op-Amp will try to equalize the potential at the positive and
the negative terminals, i.e., it will try to make
R 2
V S ≈ − ----- ⋅ V IN ,
R 1
which implies
R 2
i M ≈ − --------------- ⋅ V IN .
⋅
R S R 1
Although a current amplifier tends to have a linear relationship between the command input and the
winding current, there is practical limitation due to the limited source voltage. In Fig. 20.47, the supply
voltage is ±V. Assuming that the motor winding has resistance R M , the maximum current i MAX the voltage
source can supply is upper bounded by
V
i MAX < ------------------- .
R M + R S
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