Modern applications of information transmission, the edifying example being cable television but also spectral measurements, require the use of broadband amplifiers and stable selective performance during operation.
When we refer to VHF and UHF bands, according to ITU specifications it must be borne in mind that the lower frequency is at 50MHz, and the upper frequency reaches 900MHz, this frequency difference must be provided by the respective amplifiers.
Briefly analyzing the scheme, we observe that each amplification stage has an RC dipole of negative feedback of the collector-base voltage, and a selective negative current reaction in the emitters, the decoupling efficiency of the emitter resistor increasing with frequency.
These elements compete to obtain a relatively uniform amplification in a wide band. Depending on the transistors used and the supply voltage, this scheme may undergo changes in component values, but as a basic structure we find it in the literature in many applications.
Assemblies were experimented according to this scheme with transistors BFR90, BFR92, BFR93, BFY90 and BFS17 (SMD variant of the BFY90 transistor). Of these, the best results were obtained with transistors BFR93, and the weakest with BFY90. Miniature resistors and "chip" capacitors, SMD were used.
2. 65W amplifier
As essential elements of this amplifier are the electrical qualities and simplicity of construction. The final floor is built with MOSFET transistors, in HEXFET technology.
These transistors are controlled with bipolar transistors. Being a symmetrically constructed circuit, harmonic distortions have low values.
Power is provided with +/- 35V and a current of 2.5A. The frequency response is 1.5Hz… 125kHz with a nonlinearity of 0… 3dB; 1V sensitivity, 48 KOhm input impedance, signal-to-noise ratio ~ 99dB, power on a load of 8 Ohm is 63.
The presented assembly consists of the following transistors: BC550C, BC580C, BD139, BD140, IRF9540, IRF540.
3. Emergency relay
The main purpose of the emergency relay described below is protection of electrical equipment by disconnecting the supply voltage to which it is connected, in case a situation of defect or damage is notified within an electrical installation.
This state will be highlighted by the appearance of a voltage, U-fault, which will be transmitted to the fault relay (and which it will monitor).
Also, the relay allows both the disconnection of the protected equipment in the absence of a fault state, and the connection of the equipment to the supply voltage during a fault state, through the intervention of the human operator, who can make decisions depending on the specifics of the installation and equipment used.
For example, the decision to use equipment during a state of emergency that endangers the equipment itself may be made if the material losses caused by the equipment malfunction are greater than the cost of the equipment itself.
This is the classic case of electric motors placed in industrial installations, in locations that directly affect the production process. The emergency situation is notified in this case by the protection circuits of the motor: overload protection relay (overcurrent), thermal protection relay, etc.
The wiring diagram is given in figure 1 (from the article) and was developed around an integrated circuit from the CMOS series, namely MMC4013 (CD4013), which consists of two D-type flip-flops.
4. Master-Slave switch to the mains
The presented scheme allows the operation of a consumer powered at 220Vac (Slave) conditioned by the operation of another consumer (Master) connected to the same socket (through this scheme).
Examples of use can be multiple: activating working lights when starting the PC, opening the ambient light when turning on the TV or turning off the main light (via consumer transducer at the Slave terminal), in industrial halls, etc.
The main component is a Solid-State relay (static) with optotriac, produced by SHARP, S202S series (E2), with ZCD (zero detection).
5. Charger - automatic discharger for Ni-Cd and Ni-MH batteries
The article presents an electronic kit developed by Velleman for the periodic maintenance of Ni-Cd and Ni-MH type batteries, of different formats (R3, R6, 6F22, etc.). The assembly offers a set of voltages and charging currents of various values (hardware selectable).
By means of some jumpers it is possible to select the value of the charging current and implicitly of discharging the battery mounted at the output terminals of this kit.
The assembly is provided with a system that detects the current state of the battery under test and possibly, if it detects that it is not completely discharged, triggers the "unloading" operation before performing the actual upload.
Two charging modes are available: normal and fast. They will be referred to in a dedicated subchapter. The assembly is based on integrated circuits CD4536, LM324 and requires a continuous power supply 15V, minimum 800mA, for example from a rectifier.
6. Performance Audio Power Amplifier Series (LM3886, LM4766 and LM4780)
The article presents high fidelity audio amplifier integrated circuits, made by National Semiconductor, with SpiKeTM thermal protection (patented instantaneous dynamic protection at the temperature peaks of the final transistors in the chip, reducing thermal stress), overtemperature protection, supply overvoltage or undervoltage, overcurrents, etc.
LM3886 - High performance 68W RMS audio amplifier with MUTE function
General description:
Power: 68W RMS / 4 Ohm at Vcc = +/- 28V;
Power: 38W RMS / 8 Ohm at Vcc = +/- 28V;
Power: 50W RMS / 8 Ohm at Vcc = +/- 35V;
THD + N = 0.03% at 20Hz
Dynamics: Slew Rate = 8… 19V / us;
PSRR (Power Supply Rejection Ratio) = 85… 120dB;
Signal-to-noise ratio, SNR = 92.5dB.
LM4766 - Dual 40W RMS audio amplifier (STEREO), with MUTE function
General description:
Power: 40W RMS / 8 Ohm / channel, T variant, at Vcc = +/- 30V;
