## Description

Objectives

• To use knowledge of common-source, source-follower and common-gate amplifier configurations to design a

practical active balun circuit.

• To confirm design predictions using hand analysis and simulation tools.

• To build and test the designed balun circuit; to practice systematic laboratory troubleshooting skills.

Introduction

A balun is a circuit that converts a single-ended signal to a differential one. The single-ended signal could be generated

by a sensor or antenna. Differential signals are often better for transmitting across long wires because they cancel out

environmental interference and noise. Differential signals are also required for signal processing with differential

circuits like modulators.

Most baluns are made using passive devices – transformers, resistors, inductors, etc. An active balun uses an amplifying

device like a MOSFET or BJT. A standard MOSFET active balun circuit is shown in Fig. 1. The MOSFET device

in this circuit is biased in saturation, and works both as a common-source (CS) amplifier and as a source-follower

(SF). When considering the signal transferred from vg to vo1, we have a CS circuit. When considering the signal

transferred from vg to vo2, we have a SF circuit. To make a good balun, we want the CS and SF circuits to have

equal gain, so that the differential output is balanced.

Parts and Equipments Required

Parts

• (1) CD4007 chip.

• (4) 1µF capacitors.

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ECE 3410– Microelectronics I Lab 5: A MOSFET Active Balun Circuit

• (1) 100kΩ resistor.

• (2) 1kΩ resistors.

• (1) 300Ω resistor.

• (4) calculated resistor values.

Equipment

• Dual power supply – use to supply VDD and VG.

• Digital multimeter – use to verify bias voltage and current.

• Function generator – use to generate vin.

• Dual-channel oscilloscope – use to measure the input and output signals.

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ECE 3410– Microelectronics I Lab 5: A MOSFET Active Balun Circuit

1 Pre-Lab

Exercise 1. You will design the active balun circuit of Fig. 1 by choosing appropriate resistor values and bias

voltages. Several of the resistances and bypass capacitor values are given in Table 1. The MOSFET M1

is an N-type device from the CD4007 package. The characteristics of M1 are given in Table 2.

Active Balun Design Problem

Derive small-signal expressions for the mid-band Common-Source and Source-Follower gains of the balun circuit:

ACS ,

vo1

vg

ASF ,

vo2

vg

.

Use the small-signal equivalent circuit model shown in Fig. 2. Note that “mid-band” means you can ignore all

capacitances in this analysis. Record the derivations in your lab book, and put the expressions in your final

report.

We want the two gains to be ASF = 0.5V/V and ACS = −0.5V/V. Then the overall differential gain will be Ad =

ASF −ACS = 1.0V/V. Suppose the MOSFET has gm = 1mA/V. Solve for RS and RD under the assumption that ro is

very large (i.e. ro → ∞), and verify that the gains are ±0.5V/V. Also determine the MOSFET’s large-signal bias

current, ID (use the formula gm =

p

2µnCox (W/L)ID), and determine the gate voltage, VG, needed to generate this

current. Put all your analysis and calculations in your lab book. List your final results in a table:

Parameter Value Parameter Value M1 Operating Region (check one)

RD VS1 Linear

RS VG1 Triode

ID VD1 Saturation

VGS1 −VT0N VDS1 Cutoff

Include this table in your lab book and in your final report. You should find that M1 is operating in saturation.

Recall that saturation is where VDS ≥ VGS −VT h. If your result is not in saturation, you should re-do all your design

calculations.

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ECE 3410– Microelectronics I Lab 5: A MOSFET Active Balun Circuit

Exercise 2. Suppose the balun’s input signal is delivered from a 300Ω transmission line, and the balun is required

to act as a matched line termination, i.e. the balun’s input resistance should equal 300Ω. The NMOS

active balun circuit has a very high input impedance due to the gate of M1, which causes reflections in

the transmission line. To provide the proper input resistance, a common-gate amplifier circuit is inserted

in series with the balun’s input, as shown in Fig. 3. The common-gate circuit is designed to have a total

input impedance of 300Ω and a low gain.

Matched Impedance Design Problem

Derive expressions for the gain and input resistance of the Common-Gate stage. Using these expressions, calculate

values of R2, R3 and M2’s bias current, ID2, to satisfy the following constraints:

• gm2 = 1.5mA/V.

• ACG = 1V/V, where ACG , vg1/vs2.

• Rin = 300Ω.

Use the mid-band equivalent small-signal circuit model shown in Fig. 4. Note that in the mid-band model, the C1 acts

like a short circuit, so vd2 = vg1. The input resistance, Rin, is the effective resistance seen looking into the commongate

circuit. Derive an expression for Rin using by applying a test voltage vx to the input, as illustrated in Fig. 5. Then

solve for the total current, ix, that goes into the amplifier. The input resistance is Rin = vx/ix. Using the small-signal

model, show that the input resistance is R2 k 1/gm, and the gain (from source to drain) is approximately gmR3 (note

that R1 is ignored in this analysis because it is very large, so that R3 k R1 ≈ R3). Calculate the values for R2 and R3

that are required to yield the specified input resistance and gain.

Once your resistor values are chosen, solve the DC values of VD2, VS2 and VG2 and verify that M2 is in saturation.

Record your final expressions and calculated values in a table like this one:

Parameter Expression Calculated Value Parameter Value M2 Operating Region (check one)

Rin VGS2 −VT0 linear

ACG VS2 triode

R2 VG2 saturation

R3 VD2 cutoff

ID2 VDS2

Record all derivations and calculations in your lab book. In your final report, give the derived expressions and

calculated values in a table.

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ECE 3410– Microelectronics I Lab 5: A MOSFET Active Balun Circuit

+ VG –

vIN

VDD

C1

R1

RD

RS

C2

C2

RL

RL

+

–

vOUT

M1

vo1

vo2

CS

SF

vg1

vs1

vd1

Figure 1: A MOSFET active balun circuit.

gm vg − v ( o2 )

RS RL

RD RL

vo1

vo2

Figure 2: Small-signal mid-band equivalent circuit for the active balun.

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ECE 3410– Microelectronics I Lab 5: A MOSFET Active Balun Circuit

+ VG –

vIN

VDD

C1

R1

RD

RS

C2

C2

RL

RL

+

–

vOUT

M1

vo1

vo2

VDD

VG

R2

+

–

Common-Gate Stage

M 2

vs2

vd 2

vg1

vs1

vd1

R3 R4 = 300Ω 1µF

Figure 3: A common-gate circuit is inserted to match the balun’s input impedance.

vIN

+

–

R2

!gmvs2

vs2

R3 ! R1

vg1

Rin

300Ω

Figure 4: Small-signal mid-band equivalent circuit for the common-gate stage.

+

– R2

−gmvs2

vs2

R3 R1

vx

ix

Figure 5: Model for calculating the effective input resistance, Rin, of the Common-Gate amplifier.

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ECE 3410– Microelectronics I Lab 5: A MOSFET Active Balun Circuit

Table 1: Given Data for the Active Balun Circuit.

Component Description Value

C1 Input bypass capacitor. 20µF

R1 Input high-pass filter resistor. 100kΩ

C2 Output bypass capacitor. 1µF

RL Load resistance. 2kΩ

VDD Main supply voltage 10V

Table 2: Device data for CD4007 NMOS component.

Parameter Description Value

µNCox W

L

Scale constant. 333µA/V

2

VT0 Threshold voltage. 2.0V

Utah State University Spring 2015

ECE 3410– Microelectronics I Lab 5: A MOSFET Active Balun Circuit

Exercise 3. You should now have a complete active balun design with values for all components and bias voltages.

Create a SPICE description of the active balun circuit with Common-Gate input stage (Fig. 3). For

the input signal, use a 50kHz sinusoidal voltage source with a peak-to-peak amplitude of 1V. Perform

a transient simulation covering three periods of the input signal. Plot the single-ended input and the

differential output signals, and include these plots in your lab book and final report.

Verify that the output signals are amplitude-balanced and phase-balanced. To do this, measure the

amplitude of vo1 and vo2. If their amplitudes are equal, then the signal is amplitude balanced. If they are

not equal, record the amount of amplitude imbalance in your lab book, and explain what design

change could be made to improve the amplitude balance.

Measure the amplitude imbalance as the difference in amplitudes, in dBm:

∆A = 20log

|ACS +ASF|

0.001

,

where ACS is assumed to be a negative quantity, and ASF is assumed to be positive.

To measure the phase balance, zoom in to where the transient output curves cross zero. They should

ideally cross zero at the same time. If they don’t, measure the time difference, ∆t, between the zerocrossings.

Then the phase imbalance is given by

∆φ = 2π f∆t.

Record the phase imbalance in your lab book, and include it in your final report. Also include

your SPICE input file as an appendix to the report.

2 Physical Experiments

Procedure 1. First construct the active balun circuit of Fig. 11

. Use the bench power supply to provide VDD = 10V,

and to provide the desired gate bias voltage VG.Use the digital multimeter to precisely measure the value

of all resistances. Record these measurements in a table in your lab book.

Procedure 2. Measure the DC operating point.

Step A. Step Use the digital multimeter to measure the DC voltage at every node in your circuit.

Step B. Use the multimeter to measure the bias current, ID, by connecting the meter probes in series

between RD and VDD.

Step C. Record the measurements in a table and compare them with your design predictions.

1Caution: CMOS devices are very sensitive to electro-static discharge (ESD). To avoid damaging your components, always touch a grounded

surface before handling any MOS chips. Ideally, you should wear a grounded wrist strap when handling sensitive components.

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ECE 3410– Microelectronics I Lab 5: A MOSFET Active Balun Circuit

Step D. Re-calculate your design predictions using the measured resistor values, and add these results to

your table.

Step E. Take note of any remaining discrepancies, and try to explain them in your report.

Procedure 3. Verify the small-signal behavior.

Step A. Connect the function generator to create a 50kHz signal with a peak-to-peak amplitude of 1V.

Verify the waveform using the oscilloscope before connecting it to your circuit.

Step B. After the waveform is verified, connect it to your circuit. Use the oscilloscope probe to measure

the output waveforms vo1 and vo2.

Step C. If you are using a digital-capture oscilloscope, record the waveform and include it in your lab

book and final report. Alternatively, use a digital camera to take a snapshot of the scope display.

As a last resort, you may sketch the waveform, but be sure to include as much detail as possible.

Procedure 4. Measure the amplitude balance.

Step A. Using the oscilloscope, obtain precise measurements of vin, vo1 and vo2.

Step B. Record a table of gain values in your lab book:

ACS =

vo1

vin

ASF =

vo2

vin

Ad =

vo1 +vo2

vin

.

Step C. Do the measured results agree with the predictions? How much amplitude imbalance is observed?

• Record the measurements in a table and compare them with your design predictions.

• Re-calculate your design predictions using the measured resistor values, and add these results

to your table.

• Take note of any remaining discrepancies, and try to explain them in your report.

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ECE 3410– Microelectronics I Lab 5: A MOSFET Active Balun Circuit

Step D. Include the amount of amplitude imbalance in your final report.

Procedure 5. Measure the phase balance.

Step A. Use the zero-crossing procedure to measure the phase imbalance of your circuit.

Step B. Record this value in your lab book and include it in your final report.

Procedure 6. Build and verify the Common-Gate stage.

Step A. Disconnect the function generator from your circuit, and construct the Common-Gate circuit

of Fig. 3.Use the digital multimeter to measure the precise value of all resistors.Record these

measurements in a table in your lab book.

Step B. Measure the DC voltages and bias current of the Common-Gate stage.

• Record the measurements in a table and compare them with your design predictions.

• Re-calculate your design predictions using the measured resistor values, and add these results

to your table.

• Take note of any remaining discrepancies, and try to explain them in your report.

Step C. Connect the signal generator to your circuit as indicated in Fig. 3.

Step D. Verify the input impedance as follows. First, use the oscilloscope to precisely measure the amplitudes

of vin , vg2 and vg1.

• The ratio of amplitudes is vg2/vin = Rin/(Rin +R4).

• Using this formula and your measurement of R4, determine the value of Rin.

• Also calculate the gain ACG = vg1/vg2.

• Record this measurement in your lab book, and in your final report.

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ECE 3410– Microelectronics I Lab 5: A MOSFET Active Balun Circuit

3 Report

Your final report should be a carefully written, formal summary of your lab results. It should include the following

specific information:

• A short introduction (one or two paragraphs) to the balun circuit, identifying its applications and explaining how

it works.

• Your derived expressions for the gains and input resistance: ACS, ASF, Ad, ACG and Rin. It is not necessary to

include the details of your derivation if they are in your lab book and have been checked by the instructor or TA.

• A table reporting the DC operating point, the small-signal gains, and the input resistance of your circuit. Include

three columns:

– Your original design predictions.

– Your revised predictions based on measured resistor values.

– Your actual measurements.

• Your measured amplitude and phase imbalance, as measured from SPICE simulations and the actual circuit.

– If you observed significant imbalance, offer a reasonable hypothesis to explain why. Suggest at least one

solution to remove the imbalance.

• A plot showing your SPICE transient simulation results.

• A figure showing the oscilloscope waveform from the lab.

• Conclusions should summarize the success of your design choices and succinctly report any pitfalls that may

affect users of this circuit in the future.

• Your SPICE circuit description should be attached as an appendix.