Electronics Made Easy - a Complete Introduction to Electronics - Martin Denny [10]
It is normal to run the comparator from a single supply so the output switches between 0v and Vs-2V. An example of a comparator is shown in figure 13 where the switching point is set by voltage divider R1,R2. When the input voltage at the + input rises above the voltage set by the divider the output rises from 0V to Vs - 2V. By reversing the inputs the output would switch between Vs -2V and 0V.
LOGIC
There are two systems commonly in use, TTL (Transistor-Transistor-Logic) and CMOS (Complimentary Metal Oxide Semiconductor). CMOS allows a very high density of logic functions on one chip (often millions or even billions of transistors on a single die), and is therefore the most commonly used system in computing and other digital electronics. The systems are not directly compatible although several hybrid devices are available. If it is necessary to join the two systems the best solution would be to use a transistor in switch mode as a buffer.
This subject is discussed in the digital electronics section.
ANALOGUE CIRCUIT APPLICATIONS
OSCILLATORS
An oscillator is basically an amplifier with positive feedback. If the oscillator is driven into saturation a square wave output can be produced whilst sinusoidal oscillators operate within the range of the amplifier.
Sinusoidal Oscillators
The crystal oscillator design in fig 1 will operate from 100KHz to the upper frequency limit of the transistor. The advantage of this design is the excellent frequency accuracy and stability afforded by the crystal. The amplifier is collector to base biased and operating as an emitter follower (note amplifier gain ³ 1).
At frequencies below 100KHz where it is either impractical to use a crystal due to size and expense or where a variable frequency output is required a feedback oscillator is a better option, see figure 2.
For a variable frequency oscillator the value of R can be varied between 30KΩ and 1M03W, so use a 1MΩ double ganged potentiometer. Further variation can be achieved by range switching the capacitance value.
To produce a sinusoidal waveform the amplifier gain must be adjusted to exactly 3. At less than 3 there will be insufficient gain to produce oscillation. With the gain above 3 the peak of the waveform will start to clip.
Square Wave Oscillators
To generate a square waveform it is necessary to drive the active device ie operational amplifier or transistor into saturation. For the crystal oscillator in fig 1 best results can be achieved by adding another transistor in switch mode.
To derive a square wave output from the feedback oscillator in fig 6 the amplifier gain must be increased, ie the value of the feedback resistance increased. Fig 3 shows the output with the gain set to 6 times. The clipped sinusoidal output approximates to a square wave.
By increasing the gain further the clipped sine wave will become a square wave, see fig 4.
In many cases waveforms are not symmetrical or consist of a series of narrow pulses. Figure 5 shows a square wave where the time duration of the pulse, T1 is equal to the time of the space between the pulses T2. The Mark-Space Ratio is said to be equal, ie 1:1 or T1:T2. The repetition frequency f = 1/(T1+T2).
Figure 6 shows a square waveform with uneven Mark-Space Ratio. The mark-space ratio = T1:T2 and the pulse repetition frequency f = 1/(T1+T2).
ACTIVE FILTERS
Filters are defined as either high pass (HPF, passing high frequency signals), low pass (LPF, blocking high frequency signals), or notch which contains elements of both high and low pass, and can either pass or exclude signals within a narrow band of frequencies.
Figure 7 shows a conventional low pass filter network, combined with a non-inverting amplifier with a gain of 4.3* or 12.db see below:
G = (R4 + R3)/R3 = (33K + 10K)/10K = 4.3
To convert Gain to decibels: G = 20 Log 4.3 = 12.6db
An Approximation of Fc is calculated as follows:
Fc = 1/(2pRC) = 10^6/2*3.142*33*10^3 * 0.01 = 482Hz
Where: R = R1 = R2 and C = C1 = C2. and Fc is the cut off frequency and is defined as the frequency when the