Interface Circuits 2

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Interface Circuits 2

8 ^th Week

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2

Op Amp

 An integral part of many analog and mixed-signal system

 DC bias generation

 High-speed amplification

 Filtering etc.

 Continue to pose design challenge

 Channel length and supply voltage scale down

 Definition

 Loose definition: High-gain differential amplifier, 10

¹

~ 10

⁵

 Before

 General purpose ideal op amp

 Very high gain, high input impedance, low output impedance

 At the cost of speed, output voltage swing, and power dissp.

 Today

 Multi-dimensional compromise, optimize application specifically

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Design Parameters

 Gain

 Determine precision of feedback system

 Small-signal bandwidth

 Determines settling accuracy and closed-loop gain

 Large-signal bandwidth

 Involve nonlinear operation

 Slew-rate

 Output swing

 Differential output double the swing

 Principle challenge w/ technology scaling

 Linearity

 Noise and offset

 Determine minimum detectable signal level

 Supply rejection

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4

One Stage Op Amp

 Gain

 Less than 20 in submicron devices with typical current level

 Bandwidth

 Usually determined by C

_L

 Fig. (a)

 Mirror pole can cause stability problem

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One Stage Op Amp

 Telescopic cascode

 High gain

 Low output swing

 Additional poles

 Fig. (a): mirror pole

 Unity-gain buffer

 Limited output voltage swing

4 2

b TH out X TH

V − V < V < V + V

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6

One Stage Op Amp

 Folded cascode

 Alleviate output swing

 Less problem in shorting input and output

 Generally consume higher power

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One Stage Op Amp

 Maximum input common-mode voltage

 Fig. (a) : V_b₁-V_GS₃+V_TH₁

 Minimum input common-mode voltage

 Fig. (b): V_b₁-V_GS₃-|V_THP|

 Possible to tie input with output

 Fig. (b) allows more output voltage swing margin

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8

One Stage Op Amp

 Output voltage swing

 Minimum: V_OD3+V_OD5

 Maximum: V_DD-(|V_OD7|+|V_OD9|)

 Peak-to-peak: V_DD-(V_OD3+V_OD5 +|V_OD7|+|V_OD9|)

 Less by the overdrive voltage of tail current source

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One Stage Op Amp

 Small-signal voltage gain

 A_v = G_mR_out

 G_m ≈ g_m1

 R_op ≈ (g_m7+g_mb7)r_o7r_o9

 R_out ≈ R_op||(g_m3+g_mb3)r_o3(r_o1||r_o5)

 Lower gain than that of the telescopic cascode due to low g_m and output resistance

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10

One Stage Op Amp

 Bandwidth reduction in Fig. (b)

,( ) 3 3 1 1

tot a GS SB DB GD

C = C + C + C + C C

_{tot b}_{,( )}

= C

_{tot a}_{,( )}

+ C

_GD₅

+ C

_DB₅

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One Stage Op Amp

 Higher gain

 Lower pole at the folding point

 Lower g_m of M₃

 Higher parasitic capacitance at X

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12

One Stage Op Amp

 Folded vs. telescopic cascode

 Overall voltage swing: folded- is slightly better than telescopic cascode

 At the cost of higher power dissipation, lower voltage gain, lower pole frequencies, and higher noise

 Folded-cascode

 Input-output can be shorted together

 Input common-mode level selection is easier  can be close to supply rails

 Telescopic cascode

 DC voltage must be defined carefully

 CM levels, gate bias voltages of the PMOS and NMOS cascode transistors

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One Stage Op Amp

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14

Interface Circuits 2