Summary for the Substrate Noise Hardware Lab
Summary of the Lab and Expected Results
Prelab: The students begin with a prelab, where they examine the layout of the different transconductance amplifiers, shown in Figure 1. Just drawing the circuit from the layout is a good exercise
for the students and helps them observe some of the unique qualities of analog layout, such as the
extra transistors on either side of the pFET mirror to guarantee sufficient well doping under the
mirror pair and the common centroid layout of the input FETs. The students are then asked to
apply their classroom knowledge and predict the effects that the common centroid layout and the
cascode bias will have on the circuit operation. The students are then asked to reexamine TA2 in
terms of its proximity to the inverter array and think about the effect of the digital circuit on the
analog output. Finally, the students observe the layout with the guard ring and the wire shield and
think about the impact of these protective measures on the analog circuit.
Simple Trans Amp (TA1)
Process Resistant Trans Amp (TA2)
Figure 1: Layout of two different transconductance amplifiers. TA1 is a simple 5-transistor
trans amp. TA2 contains a cascode bias and input FETs layed out in a common centroid
configuration. These layouts are compared by the students in the prelab.
Hardware Lab: In the hardware lab, the first experiment asks the students to observe the I-V relationship of both trans amps by sweeping the positive input while holding the negative input at a
d.c. level. The students are asked to measure the positive and negative saturation current and the
voltage at which the trans amp moves into the saturated current region. Our experiments demon-
strate better matching between the positive and negative saturation currents for the amplifier with
the cascode bias. This experiments reinforces the effects of channel length modulation in the bias
transistor on the output characteristics of the trans amp.
The lab asks the students to increase the bias voltage for TA2 so that Isat will be equal for both
amps. By increasing the bias voltage, the students can obtain gain measurements that can be reasonably compared between the two amplifiers. The two parallel input transistors of TA2 sums to
the same W/L ratio as TA1; however the gain of TA1 is higher due to oxide encroachment.
The lab can be modified to use the same bias voltage for both trans amps and ask the students why
the sat current is different for each amp. This modification will help the students realize that the
bias voltage is shared between the gates of the two transistors in the cascode pair. Additionally, if
you have a current meter that is sensitive enough, the lab can be modified so that the students
measure the offset of the input voltages necessary to obtain zero output current. This will allow
the students to observe the benefit of the common centroid layout.
Vout
In the next experiment, the students observe the output voltage characteristic of both trans amps.
The positive input is connected to a triangle wave that ranges from 0V to Vdd while the negative
input is connected to a dc bias voltage. The output, left as an open circuit, is measured using an
oscope. The circuit produces the characteristic trans amp voltage curve, shown in Figure 2, with
the change in slope, indicated by the arrow. The students are asked to change the negative input
voltage and observe the change in the output curve. The knee, where the slope changes, moves
with the negative bias voltage, showing the voltage at which the negative input transistor moves
into and out of the saturation region. Pushing the negative input voltage below the common mode
input range causes a distortion of the output curve. The students can compare the voltage at which
this distortion of the output curve occurs between the two circuits, demonstrating the decrease in
operating range due to the cascode bias.
change in slope
time
Figure 2: Voltage output of the trans amp in response to a triangle wave on the positive
input.
(b)
Vout
Vout
(a)
time
time
Figure 3: Voltage output of the trans amp in response to a triangle wave on the positive
input. (a) Without guard rings and shield. (b) With guard rings and shield.
The final experiment examines the effects of shielding and guard rings on a mixed signal system.
The positive input to TA2 is still connected to the triangle wave and the negative input is connected to a dc voltage. The input to the inverter chain is connected to a 100kHz square wave. The
output of the TAs on the upper left hand corner and the lower left hand corner are measured with
the oscope and compared. The inverter chain induces noise into the trans amp output signal, as
shown in Figure 3(a). However, the guard rings and shield reduce this noise, as shown in Figure
3(b).