Performance of the Feedforward constant-V ds OTA and filter…

5.3.1 Simulation Results of Transconductor Transconductance:

Fig. 5.14 Transconductance of OTA for low power feedforward-regulated circuit

In Fig. 5.14, the proposed circuit has flatness transconductance at +0.25V~-0.25V differential input signal, and the maximum transcoductance value is about 1.75m (1/Ω).

Magnitude & Phase Response:

In Fig. 5.15 (a), the DC gain is 30.2dB and the unity gain frequency is 260MHz.

As shown in Fig. 5.15 (b), the phase margin is 84.5°.

(a)

(b)

Fig. 5.15 (a) Magnitude response (b) phase response for the transconductor

THD:

Fig. 5.16 The total harmonic distortion for the transconductor

In Fig. 5.16, the HD3 is about -44.4dB for 100MHz 600mV Vpp input signal.

CMRR & PSRR:

Fig. 5.17 The common mode rejection ratio for the transconductor without CMFF From Fig. 5.17, the value of CMRR is 3.6dB for the transconductor without

common mode feedforward, which means a large value of common mode gain for the circuit. This means common mode noise will appear in the differential output to interfere the signal.

Fig. 5.18 The common mode rejection ratio for the transconductor with CMFF

Fig. 5.19 The power supply rejection ratio for the transconductor

From Fig. 5.18, the value of the CMRR is improved from 3.6dB to 37.4dB with common mode feedforward circuit. In Fig. 5.19, there is 36.4dB PSRR at DC for the

transconductor.

5.3.2 Simulation Results of Filter Cutoff Frequency & Group Delay:

(a)

(b)

Fig. 5.20 The magnitude and group delay response for filter

From Fig. 5.20, the cutoff frequency is about 255MHz. The group delay variation is less than 5% up to 1.26 times the value of the cutoff frequency. The

maximum value of the magnitude response is not 0dB due to the drivers, which need very large current to push the DC gain of filter to 0dB.

THD/IM3:

Fig. 5.21 The total harmonic distortions for filter

Fig. 5.22 The third-order intermodulation distortions for filter

From Fig. 5.21 and Fig. 5.22, the HD3 is -43.3dB for 82.5MHz 400m input signal and the IM3 is -38.9dB for 257MHz and 253MHz input signal.

TABLE 5.2 The spec for the filter

spec

Power supply 1.8v

Filter type 4th order equiriplple linear phase filter

Cutoff frequency 255MHz

Input swing 0.4Vpp

HD3 -43.3dB @ 82MHz input signal

Group delay variation <5%@1.26fcutoff

Active area 0.3515*0.3609mm²

Power consumption 42mW

5.3.3 Measurement Results of Filter

The layout for this circuit is shown in Fig. 5.23 (a) and the die photo is shown in Fig. 5.23 (b). The active area is 0.3515*0.3609mm².

(a)

(b)

Fig. 5.23 (a) The layout for the filter (b) The die photo for the filter

The magnitude response and the group delay for the filter is shown in Fig. 5.24.

The cutoff frequency is about 250MHz, which is least requirement for the UWB, and the group delay variation is less than 5ns at the cutoff frequency.

(b)

Fig. 5.24 (a) The magnitude response for the filter (b) The group delays for the filter The following measurements express the linearity performance. The Fig. 5.25 is the THD. From this figure, the HD3 is about -40dB for 80MHz input signal. By the way, the HD2 is about -20dB, which is due to the mismatch for the current mirror pairs and mismatch for the input pair causing input offset. The second harmonic distortion maybe contributed due to the lower CMRR which is lower than the source degeneration with current feedback. As described in before, the upper band of the filter will falsely improve the HD3, so the IM3 is used to express the linearity for the upper band of the filter. In Fig. 5.26, the IM3 is shown to be about -36dB for 252MHz and 248MHz input signals.

Fig. 5.25 The total harmonic distortions for the filter

Fig. 5.26 The intermodulation distortions for the filter

The output noise is 182.7uV for the frequency from 10MHz to 250MHz as shown in Fig. 5.27.

Fig. 5.27 The output noise for the filter

TABLE 5.3 The comparison of the proposed filters with other papers Reference [34]JSSC

Frequency 100MHz 550MHz 250 250

HD3/IM3 -40dB -40dB -39.68dB -40.48dB

Output noise

level/density 700uVrms -147dBm/Hz 251.9uV (10MHz~250MHz)

182.7uV (10MHz~250MHz) Power

Consumption 86mW 140mW 42mW 42mW

Chapter 6

Conclusions

6.1 Conclusions

Since the transconductance-C filters are more suitable than switch capacitor filters in high frequency application, they are usually used in high frequency applications. However, their mainly poor are linearity, so linearity improvement techniques are required. In this thesis, there are two transconductance-C filters are proposed to apply in high frequency applications such as ultra wideband and hard-disk driver.

Source degeneration circuit must use large value of resistor to improve linearity.

Also, the current feedback works well at high frequency than voltage feedback due to the former has larger bandwidth. Hence, the proposed circuit adds negative current feedback to improve the linearity, which can achieve the requirement without large resistor. Another circuit modifying the constant drain-source voltage of input pair is proposed, which is more suitable in the lower power supply and high frequency applications. In this circuit, the common mode feedforward circuit is introduced to increase the common mode rejection ratio.

Finally, 4^th order linear phase filter by cascading two biquad sections is expressed. In the next, the simulation results and the measurement results for the

transconductors and filters are shown in the chapter 5.

6.1 Future Research

Some suggestions for the future work are given as follows. The cutoff frequency of the filter could be at 60MHz~100MHz. It has high priority over the linearity enhancement. Additionally, the tuning range should be improved, as well.

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