Attenuation-feedback tone control circuit

The picture shows an attenuation feedback tone control circuit. There are both attenuation and feedback effects in this tone control circuit
. Therefore, it is not easy to distinguish whether the high and low sounds are improved or attenuated. Next, we use the superposition theorem to
analyze the circuit.
    The superposition theorem points out that in any linear circuit, the voltage of each branch can be regarded as the algebraic sum of the voltages generated in the branch when each signal source acts separately .
Analyzing with Figure 1-79, the input signal voltage Vb of the base
can be seen as the result of two signal sources , that is, the voltage formed by the input signal voltage Vi on the base
and the output signal voltage vo pass through Negative feedback creates a voltage at the base .