Seminar

On

Distributed Amplifiers

Submitted To :

Submitted By :

Dr. Karun Rawat

Rakesh Kumar

Assistant Professor 2013EEE8274

CARE Dept.

IIT Delhi

Contents

Introduction

Basic Design Circuit of DA

Operating Principle

Analysis of DA circuit

Calculation of Optimum Number of Stages to be cascaded

References

Introduction

Distributed amplifier basic idea first introduced in 1936 [Percival 36] to overcome traditional Gain*Bandwidth limit of vacuum tube amplifiers.

The key idea is to combine parasitic capacitances of the transistors with lumped inductors to form artificial Transmission-lines.

Since transmission lines have very high cut-off frequency it enables the amplifier to achieve very wide bandwidths.

The topology of DA is suited for MMICs because its passive circuit consists of inductors which can be realized in the form of short lengths of micro strip lines.

Basic Design Circuit

Consists of a pair of input and output transmission lines coupled by transconductance of N identical MOSFETs.

The transmission lines are formed using lumped inductors and are referred to as the gate and drain lines.

Gates of transistors are connected to a transmission line having a characteristic impedance Zg with a spacing of lg .

Similarly the drains are connected to a transmission line of characteristic impedance Zd ,with a spacing ld .

Fig. Configuration of N-stage Distributed Amplifier

(Image Courtesy : Microwave Engineering, David M. Pozar)

• As the RF signal travels on the gate line, each transistor is excited by the traveling voltage wave and transfers the signal to drain line through its transconductance.

• If the phase velocities on the gate and drain lines are equal, then the signals on the drain line add in the forward direction as they arrive at the output.

• The out-of-phase wave traveling in the reverse direction will be absorbed by the drain-line termination Zd .

• The amplified signals at each stage travels towards the load.

Operating principle of DA

Analysis of DA Two separate small signal models of DA are shown one for drain line and other for gate

line

a. Transmission line circuit for the gate line of the distributed amplifier

b. Equivalent circuit of a single unit cell of the gate line

•Lg and Cg are the inductance and capacitance per unit length of the gate transmission line•Ri and Cgs/lg represent the equivalent per unit length loading due to the FET input

resistance R; and gate-to-source capacitance Cgs• Using basic transmission line theory to find the effective characteristic impedance and propagation constants of the gate and drain lines

(Image Courtesy : Microwave Engineering, David M. Pozar)

a. Transmission line circuit for the drain line of the distributed amplifier

b. Equivalent circuit of a single unit cell of the drain line

•For the drain line, the series impedance and shunt admittance per unit length are

•The characteristic impedance of the drain line can be written as

• Propagation constant

Analysis of DA…

(Image Courtesy : Microwave Engineering, David M. Pozar)

Calculation of Optimum Number of Stages to be cascaded

Power gain G can be calculated as shown When loss is present the gain of a

distributed amplifier approaches zero as N infinity This surprising behavior is explained by the fact that

the input voltage on the gate line decays exponentially as

So the FETs at the end of the amplifier receive no input signal; similarly, the amplified signals from the FETs near the beginning of the amplifier are attenuated along the drain line

The multiplicative increase in gain with N is not enough to compensate for an exponential decay for large N

For a given set of FET parameters, there will be an optimum value of N that maximizes the gain of a distributed amplifier. This can be found by differentiating with respect to N, and setting the result to zero to obtain

References

1. Microwave engineering, David M. Pozar

2. A Fully Integrated 0.5–5.5-GHz CMOS Distributed Amplifier Brian M. Ballweber, Member, IEEE, Ravi Gupta, Member, IEEE, and David J. Allstot, Fellow, IEEE

3. Distributed amplifier circuit design using a commercial CMOS process technology by Kyle Gene Ross

Thank You