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This paper presents the design of folded cascode operational transconductance amplifier (OTA). This design has been implemented in 0.18um CMOS Technology using Cadence. Spectre simulation shows that the OTA has flat gain of 47dB from 1Hz to 100 KHz

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International Journal of Applied Control, Electrical and Electronics Engineering (IJACEEE) Vol 3, No.1/2, May 2015
15
D
ESIGN OF
L
OW
P
OWER
O
PERATIONAL
T
RANSCONDUCTANCE
A
MPLIFIER FOR
B
IOMEDICAL
A
PPLICATIONS
Sarin Mythry
1
A.Gayathri
2
, Saieemah Farheen
3
, N.Jeenath
4
, M.Sowmya
5
, P.Sahith
6 1
,2
Faculty,
3,4,5,6
Graduate scholars Department of Electronics and Communication Engineering, Christu Jyothi Institute of Technology and science, Yeshwanthapur, Jangaon, Warangal Dist, Telangana State, India.
A
BSTRACT
This paper presents the design of folded cascode operational transconductance amplifier (OTA). This design has been implemented in 0.18um CMOS Technology using Cadence. Spectre simulation shows that the OTA has flat gain of 47dB from 1Hz to 100 KHz frequency, indicating stability of OTA, noise ranges as 22.49769nV/ at 10Hz to 66.89128fV/ at 1MHz and average power as 0.770mW. In this paper, we will be studying the design concepts, analysis of operational transconductance amplifier which is used for recording the bio signals. This paper plays a key role in real time applications for equipment designing of ECG, EEG, EMG, ENG devices. It is also used in recording and also for treatment of Paralysis, Epilepsy, Neuro diseases etc.,
K
EYWORDS
Folded cascode, Operational Transconductance amplifier (OTA), Cadence.
1.
I
NTRODUCTION
By the advancement in Biomedical engineering and neuroscience it became easy to extract control signals from individual and communicate their intentions through computers or prostheses. Thus bio signals can be interfaced to neural interface microsystems and monitor a large group of neurons. But the channel faced by such microsystem is low power constraints, small form factor while providing high resolution. In such systems Operational Transconductance amplifier plays a very role. They are the main building blocks of analog design but the problem is they consume more power. Basically OTA is an Operational amplifier (op amp) without any buffer. OTA is voltage controlled current source(VCCS) while op amp is voltage controlled voltage source(VCVS).OTA has a differential amplifier at the input. The main purpose of OTA is it provides current proportional to an input voltage difference. The main parameter of OTA is Transconductance i.e., the ratio of output current to input voltage. A simple first-order small signal model for an OTA is shown in Fig. 5.1. The amplifier’s transconductance is G
ma
and its frequency response in Fig. 5.1is given simply by G
ma
Z
L
. More over they are widely used because of simple design
.
International Journal of Applied Control, Electrical and Electronics Engineering (IJACEEE) Vol 3, No.1/2, May 2015
16 Figure 1.First-order model of an operational transconductance amplifier (OTA) driving a capacitive load.
There are many types of OTAs among which conventional OTAs are: Two stage OTA, folded cascode OTA, Telescopic OTA, Gain Boosted OTA and other OTAs are miller OTA, Current mirror OTA and so on.In this paper a folded cascode OTA is designed to satisfy flat gain and low power.
2.
CIRCUIT
IMPLEMENTATION
2.1 Design of Folded cascode Operational Transconductance amplifier
In this design we have used differential-input single-ended output folded cascode design. We have used here current mirrors that provide wide swing current mirrors and they are used as they provide High output impedance maximizing the DC gain Of OTA. The basic idea of folded cascode OTA is to apply cascode transistors to the input differential pair but the transistors used are of opposite type to that of input. In our design we have used differential transistors Q
1
and Q
2
as n-channel transistors and cascode transistors Q
5
and Q
6
as p-channel. This opposite type of transistors provide single gain stage amplifier same Bias voltage at output as that of input signals. Even though the folded cascode is single stage amplifier it has high gain. This high gain is due to the product of input Transconductance and output impedance. The high output impedance is due to cascode techniques used. The shown differential-to-single-ended conversion is realized by the wide-swing current mirror composed of Q
7
, Q
8
, Q
9
, and Q
10
. An important addition of the folded-cascode OTA used is the inclusion of two extra transistors, Q
12
and Q
13
. They serve two purposes. One is to increase the slew-rate performance of the OTA .During times of slew-rate limiting, these transistors prevent the drain voltages of Q
1
and Q
2
from having large transients where they change from their small-signal voltages to voltages very close to the negative power-supply voltage. Thus the inclusion of Q
12
and Q
13
allows the OTA to recover more quickly following a slew-rate condition. The main purpose of the diode-connected transistors, Q
12
and Q
13
included is to clamp the drain voltages of Q
1
or Q
2
so they don’t change as much during slew-rate limiting. A second, more subtle effect dynamically increases the bias currents of both Q
3
and Q
4
during times of slew-rate limiting. This increased bias current results in a larger maximum current available for charging or discharging the load capacitance. Here compensation is provide by the load capacitor C
L
, and realizes dominant-pole compensation. In applications where the load capacitance is very small, it is necessary to add additional compensation capacitance in parallel with the load to guarantee stability. If lead compensation is desired, a resistor can be placed in series with C
L
.
International Journal of Applied Control, Electrical and Electronics Engineering (IJACEEE) Vol 3, No.1/2, May 2015
17
The folded cascode opamp is a transconductance amplifier whose frequency response is dominated by the output pole at 1/R
out
C
L.
The bias currents for the input differential-pair transistors are equal to I
bias2
/2. The bias current of one of the p-channel cascode transistors, Q
5
or Q
6
, and hence the transistors in the output-summing current mirror as well, is equal to the drain current of Q
3
or Q
4
minus I
bias2
/2. This drain current is established by I
bias1
and the ratio of,(W/L)
3
or(W/L)
4
, to (W/L)
11
. Since the bias current of one of the cascode transistors is derived by a current Subtraction, for it to be accurately established, it is necessary that both I
bias1
and I
bias2
be derived from a single bias network.
Figure 2.A folded-cascode operational transconductance amplifier.
2.2 SMALL-SIGNAL ANALYSIS
In a small-signal analysis of the folded-cascode amplifier, it is assumed that the differential output current from the drains of the differential pair, Q
1,
Q
2,
is applied to the load capacitance, C
L
. Approximate small-signal transfer function for the folded-cascode opamp is given by (1) Here, g
m1
is the transconductance of each of the transistors in the input differential pair, and Z
L
(s)
is the impedance to ground seen at the output node. When the compensation is realized by the output capacitance only, we have (2) Where r
out
is the output impedance of OTA. This impedance is quite high, on the order of g
m
r
2ds
/2 or greater if output-impedance enhancement is used. For mid-band and high frequencies, the load capacitance dominates, and we can ignore the unity term in the denominator and thus have (3) From which the unity-gain frequency of the opamp is found to be
International Journal of Applied Control, Electrical and Electronics Engineering (IJACEEE) Vol 3, No.1/2, May 2015
18
(4) With feedback, the loop unity-gain frequency is (5) Hence, for large load capacitances, maximizing the transconductance of the input transistors maximizes the bandwidth. One more advantage of having very large transconductance for the input devices is that the thermal noise due to this input pair is reduced. since much of the bias current in folded-cascode opamps flows through the input differential pair, these opamps often have a better thermal noise performance than other opamp designs having the same power dissipation. Table 1.Transistors aspect ratios in (
MOSFETS Practical values(W/L)
NM0(Q1) 40/1.6
NM5(Q2) 40/1.6
PM2(Q3) 40/1.6
PM1(Q4) 40/1.6
PM4(Q5) 30/1.6
PM3(Q6) 30/1.6
NM1(Q7) 15/1.6
NM2(Q8) 15/1.6
NM4(Q9) 15/1.6
NM3(Q10) 15/1.6
PM0(Q11) 10/1.6
NM7(Q12) 40/1.6
NM6(Q13) 10/1.6
3.Simulation results
A folded cascode operational transconductance amplifier is designed with the design process described above and implemented in 0.18um process with 1.8 V power supply and simulated with Cadence spectre .The load capacitance is 1 pF. The parameters obtained are:
International Journal of Applied Control, Electrical and Electronics Engineering (IJACEEE) Vol 3, No.1/2, May 2015
19 Figure 3. Folded cascode OTA schematic in cadence. Figure 4.Symbol for Folded cascode OTA schematic. Figure 5.Gain of Folded cascode OTA.

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