IEEE Circuits and Systems Magazine - Q1 2020 - 25

Transimpedance Gain (dBΩ)

160
155
150
145
140
135
130
100

1K
10 K
Frequency (Hz)
CS-TIA
Inv-TIA
InvCas-TIA
RIC-TIA

100 K

CG-TIA
CR-TIA
CR-RGC-TIA
RGC-TIA

Figure 14. Frequency response of the reviewed TIAs while
targeting min power and 5 KHz BW.

FIRST QUARTER 2020

to be biased with a very small current forcing the input
transistor to operate in the weak inversion region. Thus,
open loop topologies like RGC-TIA and CG-TIA are not
a preferable option in low BW biomedical applications.
Moreover, because the input referred noise current of
the biasing current source is directly referred to the input, such topologies have the worst noise performance,
see equations (31) and (37). Since the power consumption of the cascode transistor is very low (subthreshold
operation), judging the performance by only FoM 1 can be
deceiving. In addition, the power consumption of the RGCTIA is two orders of magnitude higher than the CG-TIA
because of the added regulating amplifier. Normally, the
RGC-TIA would outperform in terms of GBW and FoM 1 if
both topologies are operating in strong inversion, see sections III-C and III-D for the 100 MHz BW simulation results.
Furthermore, equations (31) and (37) show that there
is no trade-off between power consumption and the input
referred noise of both topologies. Therefore, the same
results are stated in both of the presented minimum
noise and minimum power cases for CG-TIA and RGC-TIA.
Beside all that, the DR of the CG-TIA and the RGC-TIA is
the worst among all of the compared topologies.
B. Case 2: CPD = 10 pF, 5 KHz BW, Minimum Power
The frequency response and the input referred noise
current spectral density of the reviewed TIAs are shown
in Figs. 14 and 15 respectively. Also, a summary of the
obtained simulation results for this case is listed in
table X. Following the same comparison method used
previously, RIC-TIA and InvCas-TIA achieve the highest
GBW and the lowest power consumption at the same

Input Noise Spectral Density (pA /√Hz)

its high effective transconductance for the input transistors. Consequently, it attains the highest transimpedance
gain of the four topologies. Also, due to the high effective
transconductance of the RIC-TIA core amplifier, it has the
second lowest input referred noise at the same power consumption and scores the highest FoM 1 of the four topologies. Accordingly, the RIC-TIA is very suitable for small
input current and high sensitivity applications.
The simulation results also confirm that higher open
loop gain topologies obtain higher performance. For example, InvCas-TIA outperforms Inv-TIA and CS-TIA in
terms of GBW and input referred noise because of its superior open loop gain at the same power consumption.
Moreover, the higher the open loop gain of the core amplifier the lower the total input referred noise. Despite that
higher gain means lower I Ovl
p - p, InvCas-TIA and RIC-TIA
also achieve competitive DR and score the highest FoM 2
thanks to their low input referred noise current which
compensates the I Ovl
p - p reduction. When it comes to only
the DR, the Inv-TIA and the CS-TIA shows the superiority.
Studying the results of the CR-TIA and the CR-RGC-TIA
reveals the ability to obtain the highest transimpedance
gain at the same consumed power. This result is predicted by both equation (38b) and (44b) and is stated in section II-G In addition, the CR-RGC-TIA can achieve higher
transimpedance gain compared to CR-TIA thanks to the
regulation effect and the enhancement of the M n2 transconductance, see equation (46). However, this comes at
the cost of decreasing the dynamic range because of the
added regulating amplifier's noise.
The simulation results of the CG-TIA and the RGC-TIA
reveal that to operate in 5 KHz BW, those topologies have

1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
100

10 K
100 K
Frequency (Hz)

CS-TIA
Inv-TIA
InvCas-TIA
RIC-TIA

CG-TIA
CR-TIA
CR-RGC-TIA
RGC-TIA

Figure 15. Input noise current spectral density of the reviewed TIAs while targeting min power and 5 KHz BW.

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IEEE Circuits and Systems Magazine - Q1 2020

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