Analog Interview Question:
2. For the RC circuit shown below, if a pulse signal is given as input, then find the current I, voltage across resistor VR and output voltage Vo. What is the average value of the output waveform? Find the power dissipated in the circuit in one cycle? For very small and large RC time constant plot the output.
Ans:
Voltage Vo increases exponentially to V1 until time t1 and Voltage across R VR tries to fall exponentially to zero. When Vo reaches V1, input falls to zero so output will try to exponentially fall to zero. But before Vo falls to zero, input rises to Vm at t2. So the output settles around the average value of the input.
Average value of the output is equal to the average value of the input.
The power dissipated over one cycle is
For High RC Time constant, initially the capacitor tries to charge and discharge(but not completely) as shown in figure, but capacitor charges slowly over a period of time. Under steady state conditions the average value of the output is same as the average value of the input.
Analog Interview Question:
1. RC Circuit: For the RC Circuit shown in figure below find the current I and voltage VO across capacitor?
Here R is resistance, C is capacitance and Vi is step input with amplitude A
Here R is resistance, C is capacitance and Vi is step input with amplitude A
At time t=0, voltage across capacitor cannot change instantaneously, since the current is finite(A/R). So at t=0, output voltage is zero and increases exponentially to a voltage A at time infinity. At t=0, current is A/R and exponentially falls to zero(at infinite time) with a time constant RC.
Digital Interview Question:
1. Explain Inverter Transfer Characteristics?
Ans:
Inverter transfer characteristics are as shown in figure. Vi is input voltage in x-axis and Vo is output voltage in y-axis. As shown in figure there are five regions inverter is operating as input voltage Vi is swept from 0 to VDD. These regions are defined based on the operating regions of the MP and MN transistors.
Region R1: MN is OFF and MP is in Linear region
Region R2: MN is in ON(saturation) region and MP is in Linear region
Region R3: MN and MP both are in ON(saturation) region
Region R4: MN is in Linear region and MP is in ON(saturation) region
Region R5: MN is in Linear region and MP is OFF
When Vi reaches threshold voltage of MN(Vtn), MN enters into saturation and output tries to fall.
In the region R3 gain of the inverter is largest.
