Analog Electronics

Op Amps for Everyone, by Bruce Carter and Ron Mancini from Texas Instruments.

op amps for everyone (Texas Instrument).pdf

op amps for everyone third edition 2009 (Texas Instrument).pdf

Op-amp application notes from National Semiconductor,

An applications guide for op-amps.pdf

Single rail op-amp design from Texas Instruments

single power supply design.pdf

Various precision op-amp rectifier design.

http://sound.westhost.com/appnotes/an001.htm

Type of Op-amp circuit

1) Non-Inverting amplifier
2) Inverting amplifier
3) Unity Buffer amplifier (Voltage follower)
4) Differetial amplifier
5) Suming amplifier
6) Instrumentation ampilfier
7) Oscillator
8) Comparator
9) Threshold detector
10) Zero Level detector
11) Schmitt trigger
12) Integration
13) Differentiation
14) Rectifier
15) Logarithmic output
16) Exponential output

1) Non-Inverting amplifier Vout = (1+ R2/R1) Vin – high input impedance – low output impedance – higher bandwidth – minimum gain of 1
A resistor R1||R2 = (R1 x R2) / (R1 + R2) is inserted just before the +ve terminal will keep the input current better balanced.
The added voltage divider has introduced a voltage offset to the output signal Vout.
2) Inverting amplifier Vout = -(Rf/Rin) Vin – gain can be less than 1   When analysing the op-amp as an amplifier (ideal op-amp), the +ve and -ve is to be having the same voltage potential.
A resistor Rin||Rf = (Rin x Rf) / (Rin + Rf) is inserted just before the +ve terminal will keep the input current better balanced.
The added voltage divider has introduced a voltage offset to the output signal Vout.
The voltage divider provides a voltage level which the amplification will be based from. Signal with the same voltage level will not be shift in position, while the rest of the voltage level will be amplified.
3) Unity Buffer amplifier (Voltage follower) Vout = Vin – high input impedance – low output impedance
4) Differetial amplifier. – Poor input impedance
Voltage follower added in the front of the input to improve the input impedance. This is also similar to an instrumental op-amp.
Instrumentation amplifier.

Op-amp Selection

(cheap precision op-amp)

Precision usually means a low input offset voltage, which is quite important for voltage comparator, or amplifying small differential input signal.
Input offset <0.5mV will be consider as precision op-amp.
Input offset guide from Analog Device “MT-037, Tutorial Op-amp Input offset voltage.pdf”

Capacitor for Signal Filtering

The following article is a simplied understanding of signal filtering. Basic knowledge of signal filtering is still required before reading this section.

The simplest signal low pass filter (LPF) is presented on the right consist of a resistor and capacitor. It is commonly known as RC filter.

This RC layout is applied to circuit with low impedance input, high impedance output. The resistor will be required to complete the filter function. Signal oscillation may occured is the resistor is omitted.

One example would be LPF filtering at the output of the op-amp amplifier circuitry, where filtering is applied to the varying input signal/voltage.

The cutoff frequency of this RC filter
fc = 1/(2πRC)

R will need to be significantly small compare to the load. If the load impedence is high (infinity), then the value of R becomes not very important. If the load impedence is finite, R should be smaller than 1/10 of the load.

Click here for the calculator for the LC filter. frequency and time domain results are on the fly.
– http://sim.okawa-denshi.jp/en/CRtool.php (generate freq/time domain graph on the fly)
– http://www.2pif.com/high-low-pass-filter.php (simple calculator)

RC filter, the simplest low pass filter.

Another way of looking at the same RC filter.

Ideal analysis of the circuit
The signal in the DC or lower frequency signal can be fully transfered to the high impedence (open circuit) output, while the high frequency signal will be absorbed on the resistor (R) components.

What the high frequency signal will see:
AC signal see resistor as a load, capacitor as a short circuit, while inductor as an open circuit. High frequency signal is able to see the capacitor (C) component as a short circuit. The voltage potential of Vout is seen to be the same as the ground reference. This means that the AC signal will be completely absorbed by the resistor R component. High frequency component will not be available at Vout. They are filtered by the RC filter.

What the low frequency signal will see:
DC signal see resistor as a load, capacitor as an open circuit, while inductor as a short circuit. Low frequency signal is unable to see the capacitor (C) component well. The point Vout is seen to have a very high impedance load. This means that the DC signal will be completely transfer to the open circuit output load at Vout. The R component will be seen as small as compare to the open circuit output load. Using the voltage divider concept, most of the low frequency signal will fall on the output Vout. The low frequency signal managed to pass through the RC filter.

Please note that the above explaination is a simplfied analysis of a filter. Ideal analysis helps us to understand the circuit topology (function) at a glance without the need for detail computation. In reality, the open/short circuit represent the degree of attenuation faced by the signal. The degree of signal attenuation is dependant on the frequency of the signal and the capacitor’s capacitance.

This is another low pass filter consist of only a capacitor. This type of filter will work for current source input. Vin = Vout.

One example would be the capacitors that are found on typical dc power supply filtering at its input or output. Decoupling capacitors (100nF) that are normally found near the power input of an IC is also another example.

Capacitances required to attenuate or suppress signal of certain frequency. Please note that this formula and the table presented on the right is an approximation for filtering noise from a DC signal.

Xc = 1 / (2π f C)

C = 1 / (2π f Xc )

where Xc is the reactance of the capacitor. Xc of 1.0 for the capacitor (open circuit) is possible with lower fequency signal or lower capacitance. To attenuate the AC signal of a particular frequency, Xc has to be low with the correct capacitance implemented.

Example:

To attenuate a 50Hz signal by 10 times.

C = 1 / (2π x 50Hz x 1/10) = 31,830uF

This means that to attenuate the 50Hz component by 10 times requires about 33,000uF capacitor connected from the signal to the ground line. This capacitor will filter any frequncy >50Hz on the line.

The table on the right is a simplified guide, which recommend the capacitance to use as a low pass filter for attenuating a particular frequency.

Max frequency for capacitor (taken from “Op Amps for Everyone”)

Active filter with op-amp

For flat frequency response, use Butterworth filter

For a sharp cutoff frequency, use Chebyshev filter

For linear phase, use Bessel filter.

I didn’t realised that transistor switching speed can be so important until I had encountered a problem using it for SPI communication. The data communication gets corrupted. Go through all the codes, and eventually found that the transistor switching speed was slow. The current batch of transistor is different from my previous batch; and I always thought that all BC817 npn transistor is the same. I am wrong, it is not. The problem might have been due to my design as well, unable to discharge the base signal in time, to turn off the transistor.

Ch1(yellow) shows the signal input through a 1kohm resistor to the base of the npn transistor. Ch2(blue) is the output at the transitor’s collector terminal, with a pull up resistor of 560ohm. The is

The following present the various BC817 transistor’s switching digital speed.

Switching speed of my original transistor.

delay of about 0.7us.

using npn BC846

delay of about 2us.

using npn BC817

delay of about 2.5us.

BC817-16LT1G

delay 2us

MMBT4401LT1G

delay 4.5us

MMBTA05LT1G

delay 0.25us

Effect of the signal switching without a 10kΩ resistor across the Vbe terminal of a npn transistor BC817.

There is a slight improvement in delay, but not very noticable.

Effect of the signal switching without a 10kΩ resistor across the Vbe terminal of a npn transistor BC817.

More than 100% improvement shortening the delay, of the inverted signal by about 1us.