The number of cascades for any amplifier gets out proceeding from what coefficient of strengthening has to provide the set device. To provide strengthening coefficient 35db it is necessary to connect consistently two intensifying cascades as one cascade it is impossible to reach such strengthening.
For the most effective achievement of the demanded power we will use powerful VCh the transformer which sets such operating mode of the transistor at which it gives out the maximum power. For correction of AChH of the amplifier different receptions are used: introduction of negative feedback, application of the intercascade correcting chains. As the designed amplifier is the amplifier of power that introduction of OS involves loss of power in OS chains that reduces efficiency and therefore not expediently to apply it in this amplifier. Application of the intercascade correcting chains (ICC) considerably increases efficiency. In this amplifier MKTs of the 3rd order as it possesses good frequency properties is used.
The choice of the transistor was made in point 2, its name KT903A, is carried out according to the requirements provided in point to These requirements the KT903A transistor meets. Its main technical characteristics are given below.
At calculation the unidirectional models on VCh of output and preterminal transistors are used. There is a task: choice of the preterminal transistor. Usually he gets out approximately. As the entrance intensifying VT1 element the KT903A transistor is used
The number of dividing condensers will be equal in our case to three. Raschitayem dividing C1, C6, C11 condensers which are represented on the schematic diagram (see the Appendix. The distortions falling on each condenser will be equal:
At calculation coefficients will be used: which values undertake proceeding from the set unevenness of AChH. In our case we take their equal 77, 13, 46 that corresponds to unevenness of AChH 4db on each cascade. After find rated values Svykh.N, Lvkh.N and Rvkh.N on the following formulas:
As cascade loading on alternating current is the resistor which is switched on in a collector chain - Rk and Rn at what Rk gets out equal Rn, equivalent resistance – Rekv for which the transistor works, will be equal Rn/Then we will find output current of the I-st:
We will carry out calculation of the amplifier recognizing that distortions are distributed between cascades evenly and as only 2 cascades are used, and the general unevenness, has to be no more than 3 dB, from these conditions we find the distortions falling on 1 cascade:
At further calculation, it is necessary to choose according to what scheme the cascade will be executed: when using a throttle or active resistance in a collector chain. Let's consider both schemes and we will choose that which is most expedient for applying.
The amplifier of monitoring systems of broadcasting stations developed with use of transistors 2T947A and KT903A has the following technical characteristics: strip of working frequencies (15 MHz; coefficient of strengthening of 46 dB; unevenness of the amplitude-frequency characteristic + 1,4 dB; value of the output power of 40 W; resistance of the generator and loading of 50 Ohms; supply voltage of 25 Century.
The size of inductance of a throttle gets out so that the variable component of current was not grounded via the power supply, and the size of blocking capacity – so that the collector of the VT1 transistor on alternating current was grounded.