Q.With a circuit diagram, explain the principle of lead lag network.

A lead–lag network is a series R1–C1 combination followed by a parallel R2–C2 combination, whose output leads the input at low frequencies and lags it at high frequencies.

[!ANSWER]

At low frequencies the series capacitor blocks the signal and at high frequencies the shunt capacitor short-circuits it, so the output is maximum only at one frequency $f = \dfrac{1}{2\pi RC}$ (for $R_1=R_2=R,\ C_1=C_2=C$), where the phase angle is zero.

[!TLDR]

A lead–lag network (series R1C1 + shunt R2C2) passes maximum output with zero phase shift only at $f = 1/(2\pi RC)$; below it the output leads (lead network) and above it it lags (lag network).

A lead–lag network is a circuit in which the output voltage may either lead or lag the input voltage depending on frequency. As shown in Figure 6.3.3, it consists of a series combination of $R_1$ and $C_1$ followed by a parallel combination of $R_2$ and $C_2$; the input $V_i$ is applied at the left and the output $V_o$ is taken across the $R_2\parallel C_2$ branch.

At low frequencies the series capacitor $C_1$ offers a very high reactance and behaves like an open circuit, so almost no signal reaches the output; the small output leads the input, so the circuit behaves as a lead network. At high frequencies the shunt capacitor $C_2$ offers a very low reactance and behaves like a short circuit, so it bypasses the output to the return line; the small output lags the input, so the circuit behaves as a lag network.

At one particular frequency $f$ the output voltage is maximum. If $R_1 = R_2 = R$ and $C_1 = C_2 = C$, this frequency is

At this frequency the phase angle between output and input is zero. Thus the phase angle is positive (lead) below $f$, negative (lag) above $f$, and zero at $f$. This zero-phase, maximum-output property makes the lead–lag network the frequency-selective feedback element of the Wein bridge oscillator.

[!ANSWER]

The series $C_1$ blocks low frequencies and the shunt $C_2$ shorts high frequencies, so the output is maximum only at $f = 1/(2\pi RC)$ (equal components), where the phase angle is zero; below $f$ the output leads and above $f$ it lags — hence the name lead–lag network.