Analog & Mixed-Signal Design test (Intermediate)

Sample questions

5 sample questions from the Analog & Mixed-Signal Design test, with the answers. Samples are taken out of the live test, so you will not meet these when you sit it.

1. What is the effect of input bias current on an op-amp circuit?

   1. The input bias current increases the op-amp's gain bandwidth product.
   2. The input bias current reduces the op-amp's input impedance.
   3. The input bias current is unaffected by the op-amp's operating temperature.
   4. The input bias current flows into the op-amp's input terminals and can cause a voltage drop across the input resistor, leading to an offset voltage.

   Answer: D. The input bias current flows into the op-amp's input terminals and can cause a voltage drop across the input resistor, leading to an offset voltage.

   Op-amp input bias current flows into the input terminals and can cause a voltage drop across the input resistor, leading to an offset voltage. This effect is significant in high-impedance circuits where the bias current is not negligible. The other options describe effects related to other parameters such as offset voltage, input impedance, and gain, which are not directly caused by input bias current.

2. Which method is appropriate for measuring the common-mode rejection ratio (CMRR) of an op-amp in a realistic project scenario?

   1. Apply a differential input signal and measure the output voltage.
   2. Use a single-ended input signal and measure the output voltage.
   3. Connect the op-amp to a power supply and measure the quiescent current.
   4. Use a differential amplifier configuration with a common-mode input signal and measure the output voltage.

   Answer: D. Use a differential amplifier configuration with a common-mode input signal and measure the output voltage.

   To measure the common-mode rejection ratio (CMRR) of an op-amp in a realistic project scenario, you should use a differential amplifier configuration and apply a common-mode input signal. The CMRR is defined as the ratio of the differential gain to the common-mode gain, and it can be calculated by measuring the output voltage in response to a common-mode signal and comparing it to the differential gain. Other options are incorrect because they do not directly address the CMRR measurement methodology.

3. Which of the following statements correctly defines the Common-Mode Rejection Ratio (CMRR) of an op-amp?

   1. CMRR is the ratio of differential gain to common-mode gain
   2. CMRR is the ratio of common-mode gain to differential gain
   3. CMRR is the product of differential gain and common-mode gain
   4. CMRR is the sum of differential gain and common-mode gain

   Answer: A. CMRR is the ratio of differential gain to common-mode gain

   CMRR (Common-Mode Rejection Ratio) is defined as the ratio of the differential gain (the gain for a differential input signal) to the common-mode gain (the gain for a common-mode input signal). This ratio is a measure of an op-amp's ability to reject common-mode signals, such as noise that appears equally on both inputs. It is typically expressed in decibels (dB) and is crucial for the performance of differential amplifiers and other analog circuits. The other options are either incorrect or do not accurately reflect the definition of CMRR.

4. Which of the following statements about power supply rejection ratio (PSRR) in a CMOS OTA is correct?

   1. PSRR is only dependent on the gain of the OTA and not the power supply voltage.
   2. PSRR is a fixed value for a given OTA and does not change with frequency.
   3. Power supply rejection ratio (PSRR) is measured at the output node and depends on the frequency of the power supply ripple.
   4. PSRR is measured at the input node and is independent of frequency.

   Answer: C. Power supply rejection ratio (PSRR) is measured at the output node and depends on the frequency of the power supply ripple.

   PSRR is a measure of how well an operational transconductance amplifier (OTA) rejects variations in the power supply voltage. It is typically measured at the output node and is frequency-dependent. This is a common mistake: assuming PSRR is independent of frequency or measured at the input rather than the output. The correct statement highlights the correct measurement point and the dependency on frequency.

5. Which of the following statements accurately compares CMOS OTA PSRR with op-amp PSRR and describes when to choose each?

   1. CMOS OTA PSRR is better than op-amp PSRR in applications requiring high power supply noise rejection due to the OTA's lower PSRR and higher dependence on external components.
   2. CMOS OTA PSRR is worse than op-amp PSRR in applications requiring high power supply noise rejection due to the OTA's higher PSRR and lower dependence on external components.
   3. CMOS OTA PSRR is worse than op-amp PSRR in applications requiring high power supply noise rejection due to the OTA's lower PSRR and higher dependence on external components.
   4. CMOS OTA PSRR is better than op-amp PSRR in applications requiring high power supply noise rejection due to the OTA's higher PSRR and lower dependence on external components.

   Answer: D. CMOS OTA PSRR is better than op-amp PSRR in applications requiring high power supply noise rejection due to the OTA's higher PSRR and lower dependence on external components.

   CMOS OTAs (operational transconductance amplifiers) typically have better power supply rejection ratio (PSRR) compared to traditional op-amps because they are designed with internal compensation and have a higher PSRR due to their transconductance architecture. This makes them more suitable for applications where power supply noise is significant and needs to be rejected effectively. Op-amps, on the other hand, often have lower PSRR and may require external components to achieve similar performance.