ECE 196

Laboratory 5.1Common-Drain Amplifier

Submitted by:Donna Jane G. Ambalong

BS ECE IV

Submitted to:Engr. Olga Joy Labajo-Gerasta

August 9, 2012

I. (a)Simulation of Vin-Vout DC transfer curve of Fig. 5.1

DC Analysis of a Common-Drain Amplifier

HSPICE code:

The graph shows the transfer curve of Common drain amplifier, wherein the graph that shows the difference between Vin and Vout (level shift) without the body effect is more constant.

(b)

VIN – VOUT difference curve

HSPICE code:

The graph shows the difference curve of Vin and Vout.. It can be seen in the figure above that the graph without body effect is better compared to the graph with body effect (i.e. without body effect is more constant compared to with body effect). However, it is still not constant.

II. Simulation of the frequency response of Fig. 5.1 and the effect of body effect.

AC analysis of CD amplifier

HSPICE code:

The DC gain of the curve with body effect is -6.02 dB while the DC gain for the curve without body effect is -4.43 dB. The body effect would affect the DC gain of the circuit by decreasing it.

III. (a) Simulation of Vin-Vout DC transfer curve of Fig. 5.5

DC analysis of CD amplifier with an active load (NMOS)

HSPICE code:

The graph shows the Vin-Vout DC transfer curve of the CD amplifier with an active load. Without the body effect, the difference between Vin and Vout (level shift) is more constant.

(b)

VIN – VOUT difference curve (NMOS)

HSPICE code

The figure above shows the difference between Vin and Vout . The curve without Body Effect is almost constant.

IV. Simulation of the frequency response of Fig. 5.5 and effects the Body effect will cause

AC analysis of CD amplifier with an active load (NMOS)

HSPICE code:

From the figure above, the DC gain of the curve with body effect is -6.02 dB and the DC gain of the curve without body effect is -4.43 dB. The curve without body effect and its DC gain is almost equal to zero.

V. (a) Simulation of the Vin – Vout DC transfer curve of Fig. 5.9

DC analysis of CD amplifier with an active load(PMOS)

HSPICE code:

The figure above shows the DC transfer curve of a PMOS CD amplifier with an active load. Observe that, the function of PMOS source follower is to shift up, unlike the function of NMOS source follower which is to shift down.

(b)

VIN – VOUT difference curve (PMOS)

HSPICE code:

The figure above shows the difference between Vin and Vout of a PMOS CD amplifier without body effect.

VI. Simulation of the frequency response of Fig. 5.9 and the comparison of results with NMOS CD amplifier

AC analysis of CD amplifier with an active load (PMOS)

HSPICE code:

From the figure above, the DC gain of the circuit is -2.87 dB when VIN-DC= 0.7V.

Questions:

1. What is the circuit characteristic of common-drain amplifier? What are the advantages and drawbacks? Please list some application of CD amplifier.

Definition Expression Approximate expression

Conditions

Current gain Ai= IoutIin

∞ ∞

Voltage gain Av=VoutVin

≈1 (gmRs >>1)

Input resistance

Rin=VinIin

∞ ∞

Output resistance

Rout=VoutIout

(gmRs >>1)

This type of amplifier has the input signal fed at the gate- similar to the CS amplifier- but the output is taken at the source terminal. Also called a source-follower because the voltage at the source is approximately the same amplitude as the input (gate) voltage and is in phase with it. In other words, the source voltage follows the gate input voltage. Typically used as a voltage buffer.

2. Using the circuit of figure 5.1, 5.5, 5.9, perform the FFT analysis to get the frequency spectrum of the output waves. How about the circuit linearity?(a)

(b)

(c)

As you can observe from the figures above (FFT analyses of fig. 5.1, 5.5, 5.9 respectively) that there are no significant differences among them. Thus, they have more or less the same characteristics and all curves seem to be very linear.

3. How to increase the DC gain and the Gain Bandwidth of the common drain amplifier? Why?

By increasing the output resistance (Rs), the DC gain can be increased. The output can only be increased if the load resistance is increased, since the current remains constant from drain to source. Another way to increase DC gain is by decreasing the transconductance due to body effect gmb. The gain bandwidth can be increased by increasing the gate capacitance, which means using smaller gate widths. These points can be seen from the formula:

Av=gm R s

1+(gm+gmb ) Rs

4. Is that possible to design a common-drain amplifier that has a DC gain just equal to 0? Why?

An amplifier is supposed to be designed in such a way that it produces a useful output signal; designing an amplifier to have zero gain

does not seem to make much sense. From the formulaAv=gm R s

1+(gm+gmb ) Rs, we can say that the gain can only approach zero but will never

reach a zero value.