ULF and active bandpass filter on a quad op-amp. Encyclopedia of radio electronics and electrical engineering
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Recently, operational amplifiers (op-amps) have been widely used as preliminary stages of ULF receivers and direct conversion transceivers. Such ULFs have a high gain, low noise level and high linearity of the transfer characteristic. The appearance of microcircuits containing several op amps in one package has opened up new opportunities for ULF integration. 1401UD2 also belongs to such microcircuits, which allows you to assemble on it not only amplification stages, but also a filter, which further increases the selectivity of the transceiver.
The ULF circuit for a direct conversion transceiver is shown in Fig. 1. Amplification stages are assembled on the elements DA1.1, DA1.4, and on the elements OA1.2, DA1.3 - an active fourth-order low-pass filter with a cutoff frequency of about 2,9 kHz.
(click to enlarge)
Transistors VT1, VT2 form a push-pull emitter follower, allowing the use of VLF with a load of 50 ... 100 ohms. The ULF is easy to set up, it is enough to withstand all the values of the circuit elements and make sure that the constant voltage at the outputs DA1.1 ... DA1.4 and at the emitters VT1, VT2 is about half the supply voltage.
The ULF gain is Ku=(R4/R3)(R14/R13)=12400, it can be changed by selecting R13. The amplifier has a standard input resistance of about 1 kOhm and is designed to be connected to a passive LC low-pass filter. If necessary, the input resistance can be made larger by increasing the values of R1, R2. On the basis of 1401UD2, it is also possible to assemble a band-pass active filter, which will be the filter of the main selection of a direct conversion transceiver (Fig. 2).
(click to enlarge)
However, the transceiver still needs a simple 3rd order low-pass filter at the ULF input. To ensure high stability of the filter parameters and linearity of the transfer characteristic, all op-amps of the microcircuit are connected by voltage followers. On DA1.1, DA1.2, a fourth-order low-pass filter is assembled, and on DA1.3, DA1.4, a fourth-order high-pass filter.
The cutoff frequency of the low-pass filter is 2,9 kHz, and the high-pass filter is 320 Hz, and their values \u3b\u4bare easily changed in one direction or the other. So, in order to lower the cutoff frequency in both LPFs by n times, it is necessary to increase the values of C5, C9, C10, Sat by n times. A similar filter can be installed in V.T. Polyakov’s transceiver after preliminary VLF on transistors VTXNUMX, VTXNUMX.
The use of a filter in a microphone amplifier can significantly increase the purity of the emitted spectrum. The use of an active filter can also have an effect in those filter transceivers where the main selection is carried out using the EMF.
Author: Yu.Demin, UR5MMJ, Krasny Luch; Publication: N. Bolshakov, rf.atnn.ru
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