RC Coupled Amplifier

Dr. Hari V S
Department of Electronics and Communication
College of Engineering Karunagappally

What You will Learn

• You will understand the design of RC coupled amplifier.
• You will appreciate its frequency response and understand the variation of voltage gain at low and high frequencies.
• You will understand the effect of emitter bypass capacitor (or rather the effect of current series negative feedback) on the voltage gain and frequency response of the amplifier.
• You will understand the concept of stability of amplifier by analyzing the Nyquist plot and understand how negative feedback improves the stability.

Objectives

To design and implement RC coupled amplifier and to observe voltage amplification both in Qucs and on breadboard.

To plot its frequency response curve and to compute the bandwidth with and without feedback.

Design

Let $V_{CC}=15V$, $\beta=200$ and $I_{C}=5mA$. For active region of operation, $V_{CEQ}=\frac{V_{CC}}{2}$ \begin{equation} V_{CC} =I_{C}R_{C}+V_{CEQ}+I_{E}R_{E} \end{equation} Let the voltage across $R_{E}$ be 10 % of $V_{CC}$ \begin{align} I_{C} &\approx I_{E} \end{align} So \begin{align} \nonumber R_{E} &=\frac{0.1V_{CC}}{I_{E}}\\ \nonumber & \approx \frac{0.1\times15}{5\times 10^{-3}}\\ &= 300\,\Omega \end{align} and \begin{align} \nonumber R_{C} &=\frac{V_{CC}-V_{CEQ}-I_{E}R_{E}}{I_{C}}\\ \nonumber &= \frac{15-7.5-1.5}{5\times 10^{-3}}\\ &=1.2\,k\Omega \end{align} \begin{align} \nonumber I_{B} &\approx \frac{I_{C}}{\beta}\\ \nonumber &\approx \frac{5\times 10^{-3}}{200}\\ &\approx 25\,\mu A \end{align} Assume that $10I_B$ flows through $R_1$ and $9I_B$ flows through $R_2$. \begin{eqnarray} V_{CC}\frac{R_2}{R_{1}+R_{2}}&=& V_{BE_{active}}+I_{E}R_{E} \nonumber \\ &=& 0.8+1.5\nonumber \\ &=& 2.3\,V \end{eqnarray} \begin{align}\label{ch7eq14.eps} \nonumber R_{2} &= \frac{2.3}{9I_{B}}\\ \nonumber &= \frac{2.3}{9\times 25\times 10^{-6}}\\ &\approx 10\,k\Omega \end{align} \begin{align} \nonumber R_{1} &= \frac{V_{CC}-V_{BE_{active}}-I_{E}R_{E}}{10I_{B}}\\ \nonumber &= \frac{15-2.3}{10\times 25\times 10^{-6}}\\ \nonumber &= 51\,k\Omega\\ &\approx 47\,k\Omega \end{align} Let the coupling capacitor $C_{C}$ be $10\,\mu F$ The capacitor $C_{E}$ together with $R_{E}$ acts as a high pass filter that should bypass all ac signals above $50\,$Hz. So \begin{align} \nonumber C_{E}&=\frac{1}{2\pi R_{E} 50} \\ \nonumber &=\frac{1}{2\pi\times 300\times 50}\\ &= 10\,\mu F \end{align}

Simulation on QUCS

The Qucs schematic for RC coupled amplifier is shown below.

The transient simulation results in input and output waveforms, shown below.

The ac simulation results in the frequency response shown below. Observe the midband gain, the 3-dB frequencies and the bandwidth.

Stability of Amplifier

In simple terms, the Nyquist criterion for stability states that an amplifier is unstable if the Nyquist plot encircles the $-1+j0$ point. The Nyquist plot is the polar plot of the gain of the amplifier against frequency. Drop a polar plot on the display window and select the gain against frequency and observe the Nyquist plot. See the Nyquist plot below.

See if the amplifier is stable. Understand that the stability of the amplifier arises from current series negative feedback through the resistor $ R_E$. Change the value of this resistance and observe the change in the Nyquist plot.

Observations

Midband voltage gain of the RC coupled amplifier = $\ldots\ldots\ldots\ldots\, dB$

Bandwidth of the RC coupled amplifier = $\ldots\ldots\ldots\ldots\, kHz$

What You Learned

• You understood the design of RC coupled amplifier.
• You understood the frequency response and the variation of voltage gain at low and high frequencies.
• You understood the effect of emitter bypass capacitor (or rather the effect of current series negative feedback) on the voltage gain and frequency response of the amplifier.
• You analyzed the Nyquist plot and understood how negative feedback improves the stability.