Analog IC Designer

Analog IC Design Engineer


What does an Analog IC Designer is expected to know?

Most of the major companies will expect you to know the basics of analog circuits. If  we take an example of fresher or 1or 2 year experience , since he might not have industry experience, so companies expect him to know the basics thoroughly. So if you are a fresher, doesn’t have any IC design experience(just coming out of University), it is better to join as an intern to get some experience. Analog IC Designer is expected to know the following topics thoroughly:

Ø  RC/RL Circuits: RC/RL circuits, step response, impulse response, ramp response, response for pulse signal, steady state response, poles and zeroes, magnitude transfer function, phase transfer function, series and parallel combination of RC circuits and their step response.

Ø  RLC circuits: LC oscillation, series and parallel resonance, step response, impulse response,  Quality factor, Bandwidth, Impedance, Admittance.

Ø  MOS: current and voltage transfer characteristics, manufacturing process, moscap, dc characteristics, gm, gds, gmb, fc cutoff frequency, common source/drain/gate amplifiers, cascode amplifier, gain, bandwidth, poles and zeroes in amplifiers, source degeneration, noise, small signal analysis.

Analog Interview Questions I

2. Find the voltages Vs, Vi and Vo in the circuit below

Solution:

Current I in the circuit is constant, so voltage across capacitor increases linearly with the slope I/C. voltage across resistor R is IR. So voltage Vi in the above circuit is Vi=IR + I/C*t.

Next question is very simple 

       3. A simple RC circuit is given with an impulse voltage signal at the input, plot the voltage across resistor and capacitor. Find the current waveform?

     Solution: 

      For ideal impulse voltage signal, voltage across capacitor is zero always. But voltage across resistor is same as impulse signal and current in the circuit is similar to ideal impulse I=Vin/R.. 

      But let us think about a practical impulse signal with finite pulse 'tw' and amplitude Vm. So the voltage across resistor and capacitor at t=0+ is Vm and 0. The voltage across capacitor increase with time constant RC during pulse 'tw', for time t>tw, impulse falls to zero so the capacitor will discharge to zero from the voltage it has charge earlier.Current I in the circuit is similar to VR except it is scaled by R i.e., I=VR/R

   
      4.  Given a simple RC circuit, find the voltage across resistor and capacitor for the ramp input with a slope k.    Find the current waveform in the circuit?
      Solution:
      Initially Vo=0V, so for a given ramp input since input is zero initially Vo will be zero. How should we analyse for inputs other than step? For any RC circuit, if current is finite, voltage across capacitor cannot change instantaneously. For ramp input initial current is zero since Vi is zero at t=0, so capacitor cannot charge so Vo is zero at t=0.
      Now as input increases, current tries to increase, so voltage across capacitor also increases but slowly since current is very small initially. Voltage across resistor is VR=IR. Current increases to a point when the slope of the output is equal to slope of the input, so that current is constant and is given by (Vo-Vi)/R.

       If you solve the differential equation for the RC circuit with ramp input you will get the above equations.
       In the figure below, current waveform is not given, since it is similar to voltage VR, except scaled by R i.e,         I=VR/R

       In the next post we will discuss RL circuit, which are similar to RC circuit, but lot of people get confused whenever they see inductor. We will find simple way of analyzing RL circuits. we will discuss important points about RC/RL circuits.

http://www.ee.ucr.edu/~rlake/EE135/Fuding_Ges_Guide.html
http://stevenic.blogspot.in/2010/04/some-on-site-interview-questions-fuding.html

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Blogging Tips for Beginners: Convert Word doc to Blogger Post

I happen to listen to Darren Rowse and few others in youtube regarding blogging tips for beginners. Whatever you write should contain  Good Content. Good Content is a content which will educate reader or  solve reader's problem or entertain reader. Try to read ugly truths of blogs for beginners 

I was writing a post on a vlsi circuit which included pictures and equations. I had problems in converting a word document to blog post especially equations are not copied properly. I have found few methods to do this which might be helpful to convert word doc to blogger post

1. Open a word file, select the data you want to copy, right click and copy. Now go to edit posts in blogger and paste it where you want. This process will copy data but equations and figures or pictures are not copied. This method of converting word doc to blogger post lose figures and equations.

Voltage References Basics

Voltage reference is a reference voltage Vref stable against process, voltage and temperature. Voltage reference is a basic block used in analog/mixed signal Integrated Circuit(IC) and radio frequency ICs. Voltage reference is used in DC-DC converter, power management circuits, ADC and DAC. So there are more than one voltage reference designed in an analog/mixed signal IC. The design of voltage reference is very important because the accuracy of the voltage reference decides the accuracy and performance of the whole system.

For example, in a voltage regulator the line regulation is one of the important specifications. Line regulation refers to variation in output voltage with the variation in input supply voltages. In a voltage regulator, a voltage reference is generated and is multiplied by a specific ratio to get the regulated output voltage. So if the voltage reference doesn’t vary with the input supply voltage, then the regulated output voltage doesn’t vary with supply voltage. So the accuracy of the output voltage is decided by the accuracy of the reference voltage.

For the figure shown below, 

If VREF changes, then VOUT changes by  1+ R1/R2, so there is a gain from VREF to VOUT. If VOUT specification is 2.5V ± 1%. Then ∆VOUT = ±25mV. If VREF=1.2V and changes by 100ppm/C i.e., For change in temperature of 100C, Vref=1.2±1.2×100×1e-6×100=1.2±0.012V. Let  1+ R1/R2=2.1, then VOUT changes by 2.5±0.0252. From the above example, the accuracy of the VREF decides the accuracy of the regulator output voltage VOUT.