Penguat Differential [2]

8/18/2019 Penguat Differential [2]

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Chapter 10 Differential Amplifiers

10.3 MOS Differential Pair

10.4 Cascode Differential Amplifiers

10.5 Common-Mode Rejection

10.6 Differential Pair with Active Load


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CH 10 Differential Amplifiers 2

MOS Differential Pair’s Common -Mode Response

Similar to its bipolar counterpart, MOS differential pairproduces zero differential output as V CM changes.

2SS

D DDY X

I

RV V V


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Equilibrium Overdrive Voltage

The equilibrium overdrive voltage is defined as theoverdrive voltage seen by M 1 and M 2 when both of themcarry a current of I SS /2.

LW C

I V V

oxn

SS equil TH GS


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Minimum Common-mode Output Voltage

In order to maintain M 1 and M 2 in saturation, the common-mode output voltage cannot fall below the value above.This value usually limits voltage gain.

TH CM

SS

D DD V V I

RV 2


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Differential Response


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Small-Signal Response

Similar to its bipolar counterpart, the MOS differential pairexhibits the same virtual ground node and small signalgain.

Dmv

P

R g AV 0


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Power and Gain Tradeoff

In order to obtain the source gain as a CS stage, a MOSdifferential pair must dissipate twice the amount of current.This power and gain tradeoff is also echoed in its bipolarcounterpart.


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MOS Differential Pair’s Large -Signal Response

2211214

2

12 inin

oxn

SS inoxn D D V V

LW

C

I V V

LW

C I I in


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Maximum Differential Input Voltage

There exists a finite differential input voltage thatcompletely steers the tail current from one transistor to theother. This value is known as the maximum differentialinput voltage.

equil TH GS inin V V V V 2max21


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Contrast Between MOS and Bipolar Differential Pairs

In a MOS differential pair, there exists a finite differentialinput voltage to completely switch the current from onetransistor to the other, whereas, in a bipolar pair thatvoltage is infinite.

MOS Bipolar


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The effects of Doubling the Tail Current

Since I SS is doubled and W/L is unchanged, the equilibriumoverdrive voltage for each transistor must increase byto accommodate this change, thus V in,max increases byas well. Moreover, since I SS is doubled, the differentialoutput swing will double.

22


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The effects of Doubling W/L

Since W/L is doubled and the tail current remainsunchanged, the equilibrium overdrive voltage will belowered by to accommodate this change, thus V in,maxwill be lowered by as well. Moreover, the differentialoutput swing will remain unchanged since neither I SS nor R D has changed

22


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Small-Signal Analysis of MOS Differential Pair

When the input differential signal is small compared to4ISS / nC ox (W/L), the output differential current is linearlyproportional to it, and small-signal model can be applied.

2121214

2

1ininSS oxn

oxn

SS ininoxn D D V V I L

W C

L

W C

I V V

LW

C I I


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Virtual Ground and Half Circuit

Applying the same analysis as the bipolar case, we willarrive at the same conclusion that node P will not move forsmall input signals and the concept of half circuit can beused to calculate the gain.

C mv

P

R g A

V 0


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MOS Differential Pair Half Circuit Example I

13

31 ||||

1

0

OOm

mv r r g g A


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MOS Differential Pair Half Circuit Example II

3

1

0

m

mv g

g A


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MOS Differential Pair Half Circuit Example III

mSS

DDv g R

R A

12

2

0


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Bipolar Cascode Differential Pair

133131 || OOOmmv r r r r g g A


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Bipolar Telescopic Cascode

)||(|||| 575531331 r r r g r r r g g A OOmOOmmv


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Example: Bipolar Telescopic Parasitic Resistance

opOOmmv

OOmOop

Rr r r g g A

Rr r

Rr r g r R

||)||(2

||||2

||||1

31331

157

15755


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MOS Cascode Differential Pair

1331 OmOmv r g r g A


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MOS Telescopic Cascode

)(|| 7551331 OOmOOmmv r r g r r g g A


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Effect of Finite Tail Impedance

If the tail current source is not ideal, then when a input CMvoltage is applied, the currents in Q 1 and Q 2 and henceoutput CM voltage will change.

m EE

C

CM in

CM out

g R R

V

V

2/12/

,

,


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Input CM Noise with Ideal Tail Current


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Input CM Noise with Non-ideal Tail Current


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Comparison

As it can be seen, the differential output voltages for bothcases are the same. So for small input CM noise, thedifferential pair is not affected.


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CMRR

CMRR defines the ratio of wanted amplified differentialinput signal to unwanted converted input common-modenoise that appears at the output.

DM CM

DM

A

A

CMRR


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Differential to Single-ended Conversion

Many circuits require a differential to single-endedconversion, however, the above topology is not very good.


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Supply Noise Corruption

The most critical drawback of this topology is supply noisecorruption, since no common-mode cancellationmechanism exists. Also, we lose half of the signal.


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Better Alternative

This circuit topology performs differential to single-endedconversion with no loss of gain.


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Active Load

With current mirror used as the load, the signal currentproduced by the Q 1 can be replicated onto Q 4.This type of load is different from the conventional “staticload” and is known as an “active load”.


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Differential Pair with Active Load

The input differential pair decreases the current drawn fromRL by I and the active load pushes an extra I into R L bycurrent mirror action; these effects enhance each other.


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Active Load vs. Static Load

The load on the left responds to the input signal andenhances the single-ended output, whereas the load on theright does not.


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MOS Differential Pair with Active Load

Similar to its bipolar counterpart, MOS differential pair canalso use active load to enhance its single-ended output.


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Penguat Differential [2]

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