ETSM

In a circuit with an Operational Amplifier (OA) with nonzero offset

• the voltage gain is always larger than in the ideal OA case

• the output offset voltage is minimized if the equivalent resistances at the two OA input terminals are equal

• the output offset voltage is independent from the resistances in the circuit

• the output offset voltage is always zero

In the two-port model of a linear voltage amplifier, the input resistance

• is zero if when one changes the load value, the output voltage does not change

• is always infinite

• is always zero

• is infinite if when one changes the internal resistance of the driving generator, the output voltage does not change

The voltage comparator above has threshold voltage

• 1/5 * V_DD

• 0 V

• 4/5 * V_DD

• V_DD

Consider a Zener diode under breakdown operating conditions. The electrical behavior of the device resembles the behavior of

• A voltage generator

• a short circuit

• an open circuit

• a current generator

Answer explanation

Under breakdown operating conditions, a Zener diode behaves like a voltage generator because it maintains a nearly constant voltage which is the Zener voltage.

A p-n junction is under reverse bias when

• none of the above

• a positive potential difference is applied between p-side and n-side

• the p-side and n-side are at the same potential

• a negative potential difference is applied between p-side and n-side

A square signal with amplitude -5V to 5V and duty cycle 0.1 is measured by a TRMS voltmeter coupled in AC. What is the value expected on the display?

• about 3.5V

• about 3V

• about 7V

• about 5V but only if the frequency is 50Hz or below

Answer explanation

V_reading = √(V_rms^2 - V_dc^2)

V_rms = (V_max-V_min) * √duty_cycle = 10 * √0.1 = 3.16

V_dc = (V_max-V_min) * duty_cycle = 10 * 0.1 = 1

V_reading ~= √10-1 ~= 3

A square signal with frequency of about 65 MHz is sampled by a digital oscilloscope which has a pass band of B=2GHz and a sampler working at the frequency of 1GHz. The oscilloscope employs a sinc reconstruction filter and shows interpolated lines on the screen. The acquired signal shown on the screen is ...?

• A sine-like signal with a frequency rather different from 65MHz

• A sine-like signal with a frequency of about 65MHz

• A square-like signal with a frequency of about 65MHz

• A square-like signal with a frequency rather different from 65MHz

Answer explanation

Sinc filter reconstructs the input signal as it was. Pass band is higher than 65MHz and also sampler works significantly greater than x2 of input signal's frequency.

Usually, oscilloscopes are employed by attaching a compensated probe 1:10 at their input. If the compensated probe is a passive device:

• it increases the input resistance to more than 1GΩ

• it is used to measure signals in points difficult to reach

• it should be used when the source impedance is high

• it can be used correctly only if the source resistance is more than 100MΩ