Driving of Brushed DC Motors Using BTL Amplifier Circuits: Linear Voltage Driving

We have presented a number of examples of brushed DC motor driving methods using H-bridge circuits; we follow this with an explanation of a method for driving a brushed DC motor using a BTL amplifier circuit. In this method, the two outputs of a BTL (Bridged Transformer-Less; there are also other definitions) amplifier, which is originally a stereo amplifier, are connected to a speaker; the amplifier is used as a monaural amplifier with one output used for normal-phase driving and the other for reverse-phase driving. Here, we explain the application of this method to driving of a motor. In BTL amp driving, there are voltage driving and current driving; we begin by explaining voltage driving.

Driving of Brushed DC Motors Using BTL Amplifier Circuits: Linear Voltage Driving

The following is a BTL amp circuit that performs linear voltage driving of a brushed DC motor.

In this circuit, by controlling the DC voltages applied to the two inputs (IN1, IN2), the DC voltages (OUT1, OUT2) output to the motor are controlled, and the current direction is also controlled. The circuit example is configured with an input-stage amplifier and two output-stage power amplifiers that are connected to the input-stage amp. As indicated in the circuit diagram, the motor is connected across the two outputs.

The operation is as follows. The input-stage amp, with output M0 and inputs IN1 and IN2, is a simple differential amplifier. Therefore, M0 is the voltage obtained by multiplying the difference between the voltages applied to IN1 and IN2 by the gain determined by R2/R1 and adding Vref, as shown below.

 VM0＝(R2/R1)×(VIN2－VIN1)＋Vref

The relationships between the input M0 of the output-stage amps (= the output of the input-stage amp) and the outputs OUT1, OUT2 are shown. Similarly to the input-stage amp, the output-stage amps are differential amps, and so the relationships are as indicated below. The OUT1 amp receives M0 at the inverting input, and the OUT2 amp receives it at the non-inverting input, so that the input voltage difference terms are different.

 VOUT1＝（R4/R3）×（Vref－VM0）＋VM/2
 VOUT2＝（R4/R3）×（VM0－Vref）＋VM/2

Substituting the previous equation for VM0 into these equations and rearranging, we obtain the following.

 VOUT1＝（R4/R3）×（R2/R1）×（VIN1－VIN2）＋VM/2
 VOUT2＝（R4/R3）×（R2/R1）×（VIN2－VIN1）＋VM/2

Hence the voltage difference between OUT1 and OUT2 is as follows.

 VOUT1－VOUT2＝2×（R4/R3）×（R2/R1）×（VIN1－VIN2）

The above equation indicates that, when the voltage at IN1 is higher than that at IN2, the output at OUT2 is lower than that at OUT1, and so a current flows from OUT1 to OUT2, and in the opposite case, current flows from OUT2 to OUT1. The voltage applied to the motor is the voltage difference between IN1 and IN2 multiplied by the voltage gain, 2 x (R4/R3) x (R2/R1). Thus both forward and reverse control are possible.

When IN1 and IN2 are set to the same voltage, OUT1 and OUT2 are the same voltage (= VM/2), and so the motor can be put into a short braking state. However, if the amplifiers have offsets, the output voltage difference will not be exactly zero, and so adjustment may be necessary.

An open state is not possible with this circuit. In order to create an open state, a separate circuit must be configured.

In the next article, we will explain linear current driving using BTL amplifiers.