Download Lab 4: Supply Independent Current Source Design

Curtis Mayberry
EE435
Lab 4: Supply Independent
Current Source Design
In this lab a current mirror is designed that is robust against variations in the supply voltage. The current
mirror is required to have the following requirements:
The current mirror uses a cascoded configuration in order to protect against variations in supply voltage.
The architecture used is shown in figure 1.
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VC
VB
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Figure 1: Current Source Architecture
Design Theory and Calculations
I began my design by assigning the VEB values of each transistor. A transistor’s VEB value usually lies
between 0.2v and 0.5v for strong inversion. Before the VEB of each transistor can be assigned, the
overall voltage budget must be considered.
OR
Note: Excess Bias Voltage =
Given the constraint above, the values for each of the excess biases must be chosen wisely. We also
know that the current must be equal on both sides of the mirror (left and right) Therefore we have:
However since the source degradation resistor is used in order to the output impedance and causes the
gm with respect to Id. Also, we have:
This leaves the resistor with about 1 VEB of voltage. Also, based on our prior analysis and the circuit
architecture we know:
The standard VEB was chosen to be 0.3v, a value sufficient for strong inversion. The top two transistors,
M7 and M8 have to have higher VEB values in order to have an accurate current ratio. This can also be
improved by increasing the length of M7 and M8 from 2Lmin to 3Lmin. The middle four transistors, M3,
M4, M5, and M6 can have reduced VEB values since their excess biases are less critical.
Hence we have:
The sizing of the rest of the transistors can then be found using the square law model and IREF.
Following the transistor sizing the circuit was implemented
Implementation and Results
The current reference circuit is shown in figure 2.
Figure 2: Current reference generating circuit
After adjusting the sizes to set the VEB of each transistor approximate the specified VEB, the sizes were:
In the end R was swept in order to set the desired reference current. R was found empirically to be
28.3kΩ, very close to the original estimate.
The voltage supply independence was then tested as the power supply, Vdd was swept from 2v to 3v.
The reference voltage was plotted with respect to the reference voltage as shown in figure 3. The
reference current was able to meet the 10µA ± 0.1µA requirement even over a ±(Vdd-Vss)x10% change
in the supply voltage. (2v < Vdd < 3v)
Figure 3: Reference current variation with voltage supply variation
As you can see the low voltages supply posed a larger challenge than the high voltage supply and was
the limiting factor in the reference current variation.
After a stable current supply was established, a cascoded common source amplifier was created as
shown in figure 4 and biased. The amplifier was biased using the Vdd independent current source and
cascoded biasing circuits as shown in figure 5.
Figure 4: Cascoded common source amplifier
Figure 5: Cascoded biasing circuits (left) and cascoded common source amplifier (right)
The bias voltages are generated using cascoded current sources along with either an nmos or a pmos
diode connected transistor to generate the bias voltage for an nmos or pmos transistor, respectively. In
order to generate a specific bias voltage the size of the diode transistor is swept in order to find the size
that achieves the proper bias voltage.
The sizes turned out to be as follows according to the bias node:
Finally the current in the cascoded common source amplifier was plotted as the supply voltage was
varied from 2v to 3v as shown in figure 6. It is clear from the figure that the dc current in the common
source amplifier varies little with a varying supply voltage. The dc current varies around the 50.18 µA
nominal value from between 49.45 µA to 50.28 µA. The current variation corresponds to 1.65% change
for a 20% change in the supply voltage.
Figure 6: cascoded common source amplifier current variation with changing supply voltage
Non-cascoded comparison
Next a non-cascoded version of the current source was created by removing M3, M4, M5, and M6 as
shown in figure 7. The resistance R was then adjusted to achieve 10 µA nominal IREF at a source voltage
of 2.5v. The reference current’s supply voltage dependence was then plotted as shown in figure 8.
There is a clear degradation in supply sensitivity. The current went form being with in 0.1µA to being
within 0.5µA of the nominal reference voltage. Also note that the reference current dependence on the
supply voltage is now linear and far more sensitive to higher voltages.
Figure 7: basic (non-cascoded) current source
Figure 8: Reference Current’s supply voltage dependence
Next the effects of the increased supply voltage sensitivity on a cascoded common source amplifier
were exploered. The common source amplifier along with bias circuitry is shown in figure 9. A basic
current source was used to provide a reference current using a basic current mirror structure to a bias
voltage generating circuit utilizing a diode connected transistor. These bias voltages were then used to
bias a common source amplifier. The bias voltage on each generating circuit was then set by sweeping
the size of the diode-connected transistor until the correct bias was generated. The sizes of the diode
connected bias generation transistors are:
Figure 9: Cascoded current mirror with bias generating circuits containing basic (non-cascoded) I sources
Following the setting of the bias voltages, the current of the cascoded common source amplifier was
measured as the supply voltage was varied between 2v and 3v. The results are shown in figure 10.
Again you can see a clear degradation of the supply voltage sensitivity. The degradation in the common
source amplifier current was about the same as the reference voltage. The current through the
common source amplifier went from varying by 0.55 µA to within 3.5µA. It also interesting that the
relationship between the current and supply voltage is also now linear in the common source amplifier.
Figure 10: supply voltage dependence of the common source amplifier
Conclusion
The current source created in this lab is supply independent and has been shown to perform within
0.1µA of the nominal reference voltage even over a supply variation 20% above and below the nominal
supply voltage. A comparison was then made with a similar current mirror that was, however, not
cascoded. The dependence of the current on supply voltage was once again plotted and the
performance of the reference and amplifier current was shown to degrade significantly.