Electronics Made Easy - a Complete Introduction to Electronics - Martin Denny [11]
The gain /frequency plot of the low pass filter in figure 1 is shown in figure 8. The slope of the gain or attenuation curve is defined as attenuation per decade of frequency.
A phase shift is associated with the gain frequency plot and is typically 90 degrees at the Fc point.
If the positions of C and R (ie C1, C2, R1 and R2) are interposed then the low pass filter becomes the high pass filter shown in figure 3. Gain and Fc calculations are the same as in figure 7.
Figure 10 shows the Gain/Frequency plot for the high pass filter shown in figure 9. As before the attenuation of the filter is 42db/decade. The Fc value of 380Hz is approximately 20% less than the calculated value whereas Fc was 20% higher for the previous low pass filter.
Inspection of the plots in figures 8 and 10 show the obvious weakness in the conventional design. The amplitude instability as the filter approaches its design frequency would preclude its use in many applications although it may provide a cheaper alternative to more complicated filters.
To solve the problem of amplitude stability exhibited in the previous filters a design of the type shown in figures 11 to 14 may be adopted. The two stage low pass filter gives an attenuation of 80db per decade with no gain overshoot. Stage gain is carefully selected using precision resistors. The design frequency of the filter is calculated in the same way as the previous design.
Fc = 1/(2pRC) Where: R = R1 = R2 = R5 = R6 and C = C1 = C2 = C3 =C4
For C=0.01µF and R = 1MΩ
Fc = 106/2*3.142*1*106*0.01 = 1/0.06284 = 15.9Hz
For stage 1: K = 1.152
R3 = (K-1)R4 if R4 = 11K8Ω then R3 = 0.152*11K8Ω = 1K793Ω
The nearest preferred value to 1K79Ω is 1K78Ω, in practice several values of R4 must be tried to get the best resistor combination for the closest value of K.
For stage 2: K = 2.235
R7 = (K-1)R8 if R8 =9K31Ω then R7 =1.235*9K31Ω = 11K498Ω
The nearest preferred value to 11K498Ω is 11K5Ω.
Note in selecting values for R3, R4, R7 and R8 the operational amplifier should not be significantly loaded, and the precision resistors should not dissipate more than 1/8 W.
Component List
R1 1MΩ 0.25W MF R2 1MΩ 0.25W MF R3 1K78Ω precision MF
R4 11K8Ω precision MF R5 1MΩ 0.25W MF R6 1MΩ 0.25MF
R7 11K5Ω precision MF R8 9K31Ω precision MF
C1 to C4 0.01µF polystyrene or Silvered Mica
IC1 and IC2 CA3140
Note for low drift designs it may be necessary to select a low drift operational amplifier.
Figure 12 shows the gain frequency characteristic of the low pass filter example in figure 11. The circuit exhibits excellent gain stability and has an attenuation of 80db/decade at the design frequency. The circuit has a phase shift of 180° at Fc.
The two stage high pass filter design shown in figures 13 and 14 shares the same components as the two stage low pass filter. The calculations are therefore exactly the same.
Component List
R1 1MΩ 0.25W MF R2 1MΩ 0.25W MF R3 1K78Ω precision MF
R4 11K8Ω precision MF R5 1MΩ 0.25W MF R6 1MΩ 0.25MF
R7 11K5Ω precision MF R8 9K31Ω precision MF
C1 to C4 0.01µF polystyrene or Silvered Mica
IC1 and IC2 CA3140
A single stage inverting or non-inverting filter can also be constructed using a constant factor K of 1.58. For a non-inverting filter interpose + and – pins of the operational amplifier.
All filters of this type exhibit a slight gain in the stable region due to the K factor of the filter.
OPTICAL ELECTRONICS
Light Emitting Diodes
Gallium phosphide and gallium arsenide phosphide light emitting diodes provide a visible light source when a current is passed through them. Typically for a red LED a value of If, 5mA to 25mA will provide a sufficient light output, with the light output doubling over this range. The forward bias voltage Vf, will be of the order of 1.9V to 3V.
Generally for LED's of different colours and tri-state the current If must be increased. Figure 1 shows LED connection details.
If both the red and green LED's of the tri-state LED are illuminated the resultant colour yellow can be generated.
Displays