We proposed a new method for designing the CMOS differential log-companding amplifier which achieves significant improvements in linearity, common-mode rejection ratio (CMRR), and output range. With the new nonlinear function used in the log-companding technology, this proposed amplifier has a very small total harmonic distortion (THD) and simultaneously a wide output current range. Furthermore, a differential structure with conventionally symmetrical configuration has been adopted in this novel method in order to obtain a high CMRR. Because all transistors in this amplifier operate in the weak inversion, the supply voltage and the total power consumption are significantly reduced. The novel log-companding amplifier was designed using a 0.18
In recent years, portable and wearable personal healthcare devices have become more and more popular in the world. For these kinds of devices, the biomedical signal acquisition is an important part. The analog amplifier is one of the key building blocks to the signal acquisition unit. Therefore, several requirements such as low power, low noise, low voltage, high total harmonic distortion (THD), and high common-mode rejection ratio (CMRR) are imposed on the amplifier in the biomedical applications. The log-companding technique [
For the log-companding technique, the input signal firstly needs to be compressed from the current domain to the voltage domain by logarithmic (log) law. Then, a nonlinear signal processing can be done in the voltage domain. Finally, the processed signal is expanded back to the linear current domain from voltage domain to realize an external linear amplification. The log-companding technique is realized with MOS transistors biased in the subthreshold region for their exponential current versus voltage characteristics which consume very low power.
However, this technique has its limitations. For example, the output current range and the signal linearity are difficult to improve because of the property of MOS transistor operating in the weak inversion. To reduce the distortion, the transistors are biased with low currents which will limit the input and output current swings and reduce the signal-to-noise ratio (SNR). Although the biased currents can be improved by enhancing the widths of the transistors, it will increase the power consumption and induce a large parasitic capacitance. Because the differential operation of the conventional differential log-companding amplifier is operating in the current domain [
In this paper, a novel method is proposed to improve the performance in linearity, CMRR, and low power of the log-companding amplifier by exploiting a nonlinear function and introducing a new differential stage. This method realizes the differential operation between the input signals in the log and linear domains, which is more effective in eliminating the common-mode input signal. A novel nonlinear function is employed to improve the system linearity and extend the output linear range. This paper is organized as follows: the log-companding design technique will be introduced in Section
For the expanding technique, the input signal in current domain is firstly compressed to voltage domain. Then, a nonlinear process is performed in the voltage domain. Finally, the processed signal is expanded back to the current domain to ensure an external linear function with the input signal. The block diagram of log-companding technique is shown in Figure
Diagram of log-companding technique.
As shown in Figure
According to the EKV model [ (in weak inversion and forward saturation) and (in moderate inversion and conduction saturation).
In the conventional log-companding amplifier, all MOS transistors are biased in weak forward inversion such as in the blocks of compressor
For the PMOS operating in moderate conduction and weak forward inversion, (in moderate inversion and conduction saturation) and (in weak inversion and forward saturation).
The main purpose of every amplifier is to obtain a linearly scaled copy of the input signal at the output port. In other words, the input signal is multiplied by a gain factor to yield the output signal. For the log-companding amplifier, however, this step is accomplished by adding a gain factor to the compression signal in the log region that is translated from the input signal by the log law, which corresponds to the part of nonlinear function in Figure
The general log-companding amplifier topology (auxiliary circuitry in dashed line).
The proposed differential log-companding amplifier consists of four main parts including a compressor, a differentiator, a processor, and an expander [
The compressor is shown in Figure
The diagram of compressor.
The diagram of differentiator.
Therefore,
As shown in Figure
Suppose that the MOS transistors are biased in weak forward inversion; recalling (
The differentiator plays an important role in getting a high CMRR since the differential operation in the voltage domain.
The basic nonlinear processor is shown in Figure
Suppose that
The diagram convertor and expander.
The current bias.
According to (
From (
Multiply
Let
Equation (
Then, let us take a look at the following function:
Solving (
In most practical cases, the second term of the right hand of (
For this reason, assuming that
and combining (
From (
Recalling (
where
Although the derivation of (
The differential log-companding amplifier is designed in a standard 0.18
The output current changes from 0.23
The output current curve with a varied input current.
Transient response of output current of the amplifier.
Frequency response of the amplifier.
The distribution of THD with respect to the gain current.
Considering that THD and CMRR are influenced by the device mismatch, the result of Monte Carlo simulation is provided as shown in Figure
The Monte Carlo simulation.
The amplifier gain curve with a varied
The corner simulation results are shown in Table
The corner simulation results.
Typical | Worst case | Best case | |
---|---|---|---|
MOS model | Typical | Fast | Slow |
Temperature (°C) | 27 | 0 | 80 |
Voltage supply (V) | 0.8 | 0.8 | 0.8 |
Open loop gain (dB) | 50 | 51.7 | 46.8 |
3 dB frequency (kHz) | 510 | 340 | 850 |
Maximum input range (nApp) | 20 | 10 | 46 |
Maximum output swing ( |
6.3 | 2.58 | 10 |
THD@maximum output swing (%) | 0.028 | 0.056 | 0.03 |
CMRR (dB) | 73 | 77.8 | 64.2 |
Table
Performance comparison with other works.
This work | [ |
[ |
[ | |
---|---|---|---|---|
Technology | 0.18 |
0.18 |
0.25 |
— |
Topology | Current mode | Current mode | Current mode | Current mode |
Supply voltage (V) | 0.8 | 1 | 0.6 | 1 |
Open gain (dB) | 50 dB | 44.5/50/55.9 | −40 to 38 | −40 to +40 |
Bandwidth (Hz) | 510 k | 0.3~1 k~10 k | 200 k | — |
Input referred noise density (fA/sqrt (Hz)) | 166.1 fA (@10 kHz) | 153 fA (@10 kHz) | — | — |
Maximum input current (nApp) | 20 | 20 | — | — |
Input dynamic range (dB) | 92.9 | 53.29 | — | — |
Power consumption ( |
6 | 13 | 3.16 | 25 |
CMRR (dB) | 74 | — | 35.76 | — |
THD@maximum input (%) | 0.0287@50 dB | 1.03 | 0.55 | 0.6 |
A novel high linearity, low power, and high CMRR differential log-companding amplifier is introduced in this work. The amplifier is designed in a standard 0.18
The authors declare that there is no conflict of interests regarding the publication of this article.
This study was funded by the National Basic Research Program 973 (no. 2010CB732606), National High-Tech R&D Program of China (863 Program, no. SS2015AA020109), the National Natural Science Foundation of China (no. 61401453), the STS Key Health Program of Chinese Academy of Sciences (nos. KFJEW-STS-097 and KFJ-EW-STS-095), the Guangdong Innovation Research Team Fund for Low-Cost Healthcare Technologies in China, the External Cooperation Program of Chinese Academy of Sciences (no. GJHZ1212), Guangdong Science and Technology Project for Application Research and Development (no. 2015B010129012), the Key Lab for Health Informatics of Chinese Academy of Sciences, the Peacock Program to Attract Overseas High-Caliber Talents to Shenzhen, and Shenzhen Municipal Government (nos. CXZZ20150504145109589, JCYJ20150630114942270, JCYJ20140417113430655, and JCYJ20140417113430619).