Issue link: https://resources.system-analysis.cadence.com/i/1325428
RF Electronics Chapter 10: Operational Amplifiers Page 354 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. For the non-inverting amplifier, Rg = 10 is a good compromise. Rf sets the gain and Rf = 10 k results in a gain of G = (Rf+ Rg)/ Rg = 1001. Some bias current needs to flow into the non-inverting OpAmp input, so that a resistor Rt is required. Rt should be large enough, not to load the input and small enough not to cause any DC offset voltage at the output due to the OpAmp bias current. Rt should be > 1 k to prevent loading the microphone and small enough to ensure that the input bias current does not cause an offset voltage. The source impedance Rs and Rt form a potentiometer, which reduces the input signal. Rt = 100 k results in a 0.6% voltage drop across this potentiometer and a total amplifier gain of 995, which is close enough to the desired gain. Equation 10.17 shows that the value of Rt has little effect on the amplifier noise output since Rs/(Rs+Rt) is very small. Figure 10.18. Inverting amplifier. For the inverting amplifier, a minimum value of Rg = 1 k, is required to prevent loading the microphone. Since Rs = 600 Ω and Rg = 1kΩ, for a gain G = Rf/ (Rs+Rg) = 1000, a value of Rf = 1.6 M is required. Resistor Rt must now be small to not affect the noise at the output. Rt = 10 is suitable. Figure 10.18 shows the resulting circuit. Equations 10.19 and 10.21 to 10.27 determine the noise contributions of each of the noise sources in the inverting amplifier. For the inverting amplifier, the noise power at the output of the OpAmp due to each component is as follows: OpAmp noise voltage � �� � � �� � � �� � �� � � � � � Eqn. 10.21 OpAmp noise current +in �� ��� �� � � � �� � � �� � �� � � � � � Eqn. 10.22 OpAmp noise current -in �� ��� �� �� � �� � �� � � � �� � �� � � � �� � � � � � � Eqn. 10.23 Resistor R s noise � ������ � � � � � � �� � � � Eqn. 10.24 Resistor R t noise � ������ � � � � �� � �� � � � �� � � � Eqn. 10.25 Resistor R g noise � ������ � � � � � � �� � � � Eqn. 10.26 Resistor R f noise � ������ � Eqn. 10.19 RF Electronics: Design and Simulation 354 www.cadence.com/go/awr