Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Low Noise Amplifiers - I

Prof. Bhaskar Banerjee

EERF 6330- RF IC Design

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Outline

LNA Considerations NF, Gain, Linearity, Stability, Bandwidth

Input Matching Basic LNA Topologies

Reading: RF Microelectronics, B. Razavi

2

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

NF Considerations The NF of the LNA directly adds to the RX noise figure. A typical LNA has a NF of 2-3 dB.

3

If RS = 50 , a very small value!

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Effect of Metal Resistance Consider an LNA that connects its input to a pad through a metal line 200 m long.

In order to minimize the input capacitance, the width of the line is chosen to be 0.5 m. Assuming a noise figure of 2 dB for the LNA and a sheet resistance of 40 m/ for the metal line, determine the overall noise figure. Neglect the input-referred noise current of the LNA.

4

We draw the equivalent circuit as shown in figure below, pretending that the line resistance, RL, is part of the LNA. We thus write

where NFLNA denotes the noise figure of the LNA without the line resistance. Since NFLNA = 2 dB 1.58 and RL = (200/0.5) 40 m/ = 16 , we have

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Gain of the LNA

The gain of the LNA must be large enough to minimize the noise contribution of subsequent stages, specifically, the downconversion mixer(s).

5

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Input Return Loss

The quality of the input match is expressed by the input return loss, defined as the reflected power divided by the incident power. For a source impedance of RS, the return loss is given by:

6

Constant contours in the Zin plane. Each contour is a circle with its center shown.

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Stability

Stern stability factor, defined as:

Unconditionally stable: K > 1 and < 1 for all frequencies. Note: = S11S22 - S12S21

In modern designs, the load impedance of the LNA is relatively well controlled making K a pessimistic measure of stability

Also, since the LNA o/p is typically not matched to the input of the mixer, S22 is not very meaningful.

7

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Stability Example

Consider a cascade stage with a high reverse isolation (S12 0). If the output impedance is relatively high (S22 1), determine the stability considerations.

8

In other words, the forward gain must not exceed a certain value. For < 1, we have

implying that the input resistance must remain positive.

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Linearity

In most cases, the LNA does not limit the linearity of the receiver.

But in FDD systems (e.g. LTE) - LNA linearity is critical!

9

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Bandwidth

The LNA must provide a relatively flat response for the frequency range of interest, preferably with less than 1 dB of gain variation. The LNA -3-dB bandwidth must therefore be substantially larger than the actual band so that the roll-off at the edges remains below 1 dB.

10

An 802.11a LNA must achieve a -3-dB bandwidth from 5 GHz to 6 GHz. If the LNA incorporates a second-order LC tank as its load, what is the maximum allowable tank Q?

Q of the tank must remain less than 5.5 GHz/1 GHz = 5.5

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Band Switching in LNAs

Bands can be switched by adding or removing a capacitor to the LNA tank - useful technique to achieve a relatively large fractional bandwidth.

11

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Input of the CS Amplifier

Typically the input of the LNA needs to provide a 50 impedance to match to the antenna/filter.

12

Typical values:CF = 10 fF, CL = 30 fF, gmRD = 4 and RD = 100 .At 5 GHz: Re {Yin} = (7.8 k)-1, far away from (50 )-1.

Mainly owing to very little feedback through CF at these frequencies!

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Resistive Matching at the Input Use a resistor (RP) at the input to create the 50 matching.

13

M1 and RD provide the required noise figure and gain. RP is placed in parallel with the input to provide Re {Zin} = 50 an inductor is placed between the RS and the input to cancel Im {Zin}.

RS = RP -> NF = 3 dB + circuit noise!

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Overview of LNA Topologies

14

Common-Source Common-Gate Broadband Topologies

Inductive Load Resistive

Feedback Cascode, Inductive

Load, Inductive Degeneration

Inductive Load Feedback Feedforward Cascode and

Inductive Load

Noise-canceling LNAs

Reactance-canceling LNAs

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

CS with Inductive Load Supply voltage scales down with process (CMOS channel length). For

example, at low frequencies,

15

To circumvent the trade-off expressed above and also operate at higher frequencies, the CS stage can incorporate an inductive load.

Can operate with very low supply voltages

L1 resonates with the total capacitance at the output node, affording a much higher operation frequency than does the resistively-loaded counterpart

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

CS with Inductive Load: Input Matching

Redraw: RS models the inductor loss. Tank impedance:

16

Voltage drop:

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

CS with Inductive Load: Input Matching

Substituting ZT:

17

It is thus possible to select the values so as to obtain Re{Zin} = 50

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

CS with Inductive Load: Input Matching The feedback capacitance gives rise to a negative input resistance at

other frequencies, potentially causing instability.

18

The numerator falls to zero at a frequency given by

Thus, at this frequency (if it exists), Re{Zin} changes sign.

It is possible to neutralize the effect of CF in some frequency range through the use of parallel resonance.

Will introduce significant parasitic capacitances at the input and output and degrading the performance.