0 0.5 1 1.5 2 2.5 3 3.5 4 Time (µs)
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
Voltage (V)
T1 T2 T3 T1 T2 T3
2.55 2.6 2.65 2.7 2.75 2.8 2.85 2.9 Time (µs)
0.1 0.2 0.3 0.4
Voltage (V)
2.05 2.1 2.15 2.2 2.25 2.3 Time (µs)
0.1 0.2 0.3 0.4
Voltage (V)
Falling edge (a)
(b)
Rising edge
Figure 3.13: Simulated large-signal response (a) and its zoomed portion (b) of the multifinger OTA with STI effect (blue) and without STI effect (red).
Table 3.7: Simulated two-stage OTA performance parameters for the test cases mentioned in Table. 3.3
OTA Parameter Value
Results
T1 T2 T3
D P D P D P
DC-gain (dB) >40 43 43.5 43 43.5 43 43.5
fu (MHz) >4 5 4.1 5.6 4.12 5.9 4.15
Phase margin (deg.) >60 57 58.2 55 58.5 54 59 Slew rate (V /µs) >2 2.3 2.06 2.7 2.07 2.88 2.1
Power (µW) minimum 4.9 3.27 5.2 3.28 5.3 3.3
D: Direct scaling without STI effect consideration; P: Proposed method with con- sideration of STI effect
3.7. The DC-gain mainly depends on the channel length, hence remains almost same for all the three cases. However, in direct scaling method, the UGF and slew rate change with NF due to variation in drain current caused by STI effect. The proposed method considered the STI effect (i.e., variation in NF) during the design of OTA, hence the UGF and slew rate are nearly constant. Moreover, in direct scaling, the OTA achieves the targeted specifications but it consumes more power as compared to proposed method. In case of T3 in Table. 3.7, the OTA consumes 40% additional power because of STI effect.
Fig. 3.14 illustrates the simulated open-loop output voltage characteristics of two-stage OTA for different cases. From the simulation it can be noted that, the circuit becomes non-linear with direct scaling due to the fact that the output DC operating point moves towards one of the supply rails (
3.8 Simulation Results
−20 −1.5 −1 0.5 0 0.5 1 1.5 2
0.1 0.2 0.3 0.4 0.5
Vid (mV) Vo (V)
−20 −1.5 −1 −0.5 0 0.5 1 1.5 2
0.1 0.2 0.3 0.4 0.5
Vid (mV) Vo (V)
T1 T2 T3
T1 T2 T3
−0.025 0 0.025 0.15
0.2 0.25
−0.025 0 0.025 0.25
0.255 0.26 0.265 Proposed scaling
Direct scaling
Figure 3.14: Simulated DC open-loop output voltage characteristics of OTA with respect to differential input voltage.
VDDor GND). This results in limited output swing under open-loop conditions and generates distorted output, i.e., clipping occurs in output signal with direct scaling. In the proposed scaling, the two-stage OTA provides the maximum output swing for all test cases since the output bias voltage is always close to its initial bias point i.e., at mid of supply voltage.
(a) (b)
20.8µm
25.4µm
21.3µm
19.5µm
Figure 3.15: Layout of two-stage OTA: (a) with multifinger MOSFETs using direct scaling and (b) with multifinger MOSFETs using proposed scaling
The layout of two-stage OTA for the test case T1 is shown in Fig. 3.15. The OTA layout based on multifinger devices with direct scaling and proposed scaling is shown in Fig. 3.15 (a) and (b), respectively. The area of two-stage OTA with multifinger MOSFETs using proposed scaling method
is 20 % less compared with direct scaling.
The magnitude and phase response of bulk-driven OTA is shown in Fig. 3.16. It is clear from Fig.
3.16 that, in direct scaling method, there is a variation in the responses of T1, T2 and T3 test cases.
Whereas the proposed design exhibits approximately similar responses and the deviation is negligible.
Hence, it is clear from Fig. 3.16 that the STI effect is minimized in the bulk-driven circuit using the proposed design methodology.
-30 -20 -10 0 10 20
Magnitude (dB)
T1 T2 T3 T1 T2 T3
101 102 103 104 105 106
Frequency (Hz) -200
-150 -100 -50 0
Phase (deg.)
0.9 1.1 1.3 1.5
×105 -0.5
0 0.5
0.9 1.1 1.3 1.5
×105 -110
-100 -90
Figure 3.16: Simulated magnitude and phase response of bulk-driven OTA with STI effect (blue) and without STI effect (red).
Fig. 3.13 (a) illustrates the transient response of the bulk-driven OTA with STI effect (blue waveform) as well as the direct scaling without STI effect (red waveform). Fig. 3.13 (b) shows the zoomed portion at the rising edge and the falling edge of output. It is evident from Fig. 3.13 (b) that the proposed method minimized STI variation in the transient response.
The open-loop output voltage characteristics of bulk-driven OTA for different cases is shown in Fig. 3.18. It is clear from Fig. 3.18 that the bulk-driven circuit becomes non-linear with direct scaling due to shift in the output DC bias point. However, in the proposed scaling, the output bias voltage is always closer to its initial bias point in all cases.
The performance parameters of bulk-driven OTA obtained using the proposed and direct scaling methods are listed in Table. 3.8. It is clear from Table 3.8 that the performance varies with changes in direct scaling method. Moreover, the bulk-driven OTA with direct scaling consumes more power because of STI effect.
3.8 Simulation Results
0 50 100 150 200 250 300
Time (µs) 0
0.1 0.2 0.3 0.4 0.5
Voltage (V)
T1 T
2 T
3 T
1 T
2 T
3
110 111 112 113 114 115 Time (µs)
0 0.1 0.2 0.3 0.4 0.5
Voltage (V)
161 162 163 164 165 166 Time (µs)
0 0.1 0.2 0.3 0.4 0.5
Voltage (V)
Rising edge Falling edge
Figure 3.17: Simulated large-signal response (a) and its zoomed portion (b) of the multifinger bulk-driven OTA with STI effect (blue) and without STI effect (red).
-50 -40 -30 -20 -10 0 10 20 30 40 50
Vid (mV) 0
0.1 0.2 0.3 0.4 0.5
Vo (V)
T1 T
2 T
3
-50 -40 -30 -20 -10 0 10 20 30 40 50
Vid (mV) 0
0.1 0.2 0.3 0.4 0.5
Vo (V)
T1 T
2 T
3
-1 0 1
×10-3 0.22
0.24 0.26
-5 0 5
×10-4 0.24
0.25 0.26 Direct scaling
Proposed scaling
Figure 3.18: Simulated DC open-loop output voltage characteristics of bulk-driven OTA with respect to differential input voltage.
Fig. 3.15 illustrates the layout of bulk-driven OTA for the test case T1 in Table 3.6. The layout of bulk-driven TOA based on multifinger devices with direct scaling and proposed scaling is shown in Fig. 3.15 (b) and (c), respectively. The area of OTA with multifinger MOSFETs using proposed scaling method is 8 % less compared to direct scaling.
Table 3.8: Simulated bulk-driven OTA performance parameters for the test cases mentioned in Table. 3.6
OTA Parameter Value
Results
T1 T2 T3
D P D P D P
DC-gain (dB) >15 16.91 17 16.96 17.1 17 17.2 fu (kHz) >100 145.9 99.2 132 99.4 119.8 98.7 Phase margin (deg.) >60 75 84 77.8 83.5 79 83.5 Power (µW) minimum 2.56 1.55 2.22 1.55 1.97 1.54
D: Direct scaling without STI effect consideration; P: Proposed method with consideration of STI effect
106µm
37µm
106µm
31µm
(a)
(b)
Figure 3.19: Layout of bulk-driven current mirror OTA: (a) with multifinger MOSFETs using direct scaling and (b) with multifinger MOSFETs using proposed scaling.