Sunday, July 18, 2010
The are many different types of active filters including high pass, low pass, band reject and there are numerous responses including multiple feedback band pass (MFBP), dual-amplifier band pass (DABP) and, state variable bi-quad all pole circuits. Interestingly all known filter responses such as Butterworth and Chebyshev may be synthesised.
The filter bandwidth (BW) is the difference between the upper and lower pass band frequencies. The quality factors, or Q of the filter is a measure of the distance between the upper and lower frequency points and is defined as (Center Frequency / BW) so that as the pass band gets narrower around the same center frequency, the Q factor becomes higher. For a single op-amp band pass filter with both capacitors the same value, the Q factor must be greater than the square root of half the gain, so that a gain of 98 would require a Q factor of 7 or more.
With this circuit the op-amp will turn on the LED if the input voltage out of limits, the two 1N4148 diodes to form an “AND”-gate at the output. Input voltage is to be monitored are fed through a series of 10k resistors on the input of both op-amps. If the input voltage is greater than the limit set by V1 it will CA3140 output swing to almost full supply voltage and LED light. Similarly, if the input voltage is less than the limit set by V2 the op-amp will swing to the Vcc and the LED light.
Because we can’t reduce input capacitance of this circuit, so we use the 2N4416 which has low capacitance. It is operated as a source follower with bootstrapped gate bias resistor and resistor . [Source: National Semiconductor Application Note]
This preamplifier has low output impedance, and is designed to drive long cables, allowing you to listen to a remote music source without having to buy expensive screened cables. The very low output impedance of around 16 ohms at 1KHz, makes it possible to use ordinary bell wire, loudspeaker or alarm cable for connection. The preamplifier must be placed near the remote music source, for example a CD player. The cable is then run to a remote location where you want to listen. The output of this preamp has a gain of slightly less than one, so an external amplifier must be used to drive loudspeakers.
The LM35 of National Semiconductors that is used in this project is a precision centigrade temperature sensor, which has an analog output voltage. It has a range of -55ºC to +150ºC and an accuracy of ±0.5º C. The output voltage is 10mV/ºC. The output voltage is converted by the AD convertor of the AT Mega8. The temperature is displayed on an LCD module. This is the figure of the circuit.
In this example the thermometer has a range of 0ºC to 40ºC and a resolution of 0.5ºC. If you want to have a read out in Fahrenheit you can use the LM34. The software for this project is written in BASCOM AVR. The BASCOM AVR compiler has build in commands for reading out the ADC port of a AVR microcontroller. The result is displayed on a LCD module in a discrete value of the temperature and in a bar-graph. The AT Mega8 has a A/D converter which can give an output of 210 = 1024 discrete values. When a 5V supply is used you have a resolution of 5000mV/1024 = 4.8mV. Because the LM35 has a output of 10mV/C the resolution of the thermometer is 10mV/4.8mV ~ 0.5ºC. The LCD module has 20 columns. In the scale of 0ºC to 40ºC every column represents 2ºC. [Circuit’s Source: National Semiconductor, Inc].
The input battery voltage is raised by a factor of 10 across the transformer and further raised by a voltage triple consisting of three capacitors and diodes connected to the high voltage side of the transformer. The circuit draws about 40 milliamps and should operate for about 200 hours on a couple of 'D' alkaline batteries. Higher voltages can be obtained by reducing the 4.7K bias resistor.