What does the project present?
The electronic DIY kit presented is intended to familiarize you with the performance and construction preamplifiers with low input impedances. The realized assembly can be used as an amplifier for measuring devices, in automations, picup with ceramic dose, etc.
How does the assembly work?
Specific to this preamplifier is bootstrap connection. The role of the connection is to achieve a high input impedance for the introduction of a positive reaction in a circuit provided with a negative reaction, while maintaining a good thermal stability.
The input impedance of the assembly is reduced due to the existence of the polarization resistors R1 and R2 placed in parallel. To reduce their influence, between the base of T1 and their common point, the resistor R3 was introduced.
By means of capacitor C3, a voltage is brought to the input in phase with the output voltage (positive reaction). Assuming the reaction voltage is a fraction of the input voltage (kx U1), a voltage equal to (3-k) x U1 will appear at the terminals of resistor R1. From the point of view of the signal source, the circuit behaves as if the resistor R would be 1/1 (1-k) times higher.
For example, for k = 0.98 bootstrap connection ensures a 50-fold increase in resistance. In this way the input impedance can be increased from a relative value of 47 KOhm to values exceeding 470 KOhm.
This DIY electronic kit is designed to work with relatively higher input voltages and therefore the amplification could be reduced from 250 (SME 8915) to about 80. In this way an improvement of stability is obtained eliminating the need to introduce the 10pF capacitor. between the collector and the base of the first floor. The frequency band extends up to 80 KHz.
Technical characteristics of the assembly
- Rated output voltage: min. 3V
- Frequency band (2.2 dB): 40… 80000 Hz
- Distortions: max. 0.5%
- Input impedance: min. 470 KOhm
- Current absorbed from the source: max. 10 mA
- Signal to noise ratio: min. 52 dB
- Nominal sensitivity: 2 mV
Maximum undistorted signal which we can obtain at the output and therefore the maximum signal allowed at the input, depend on the choice of the supply voltage:
List of required components (with recent equivalents):
- R1 - resistor 180 KOhm (min. 25W)
- R2 - resistor 470 KOhm (min. 25W)
- R3 - resistor 100 KOhm or 120 KOhm (min. 25W)
- R4 - resistor 82 KOhm (min. 25W)
- R5 - resistor 27 KOhm (min. 25W)
- R6 - resistor 1 KOhm (min. 25W)
- R7, R8 - resistors 100 KOhm or 120 KOhm (min. 25W)
- R9 - resistor 3.3 KOhm (min. 25W)
- C1, C3 - capacitors 2.2 uF / 63V
- C2, C5 - capacitors 4.7 uF / 40V
- C4 - 10 uF / 25V capacitor
- T1 - transistor BC 173 (BC 413, BC 109) or BC 184 / BC 550
- T2 - transistor BC 253 (BC 415, BC 179) or BC 559
- T3 - transistor BC170 or BC 546 / BC 547 / BC 548
- Printed wiring or breadboard test board
- Tin or connecting threads
Download the original IPRS Baneasa SME-8903 leaflet
For a better understanding of the operation of the circuit we will need electronic scheme presented below:
In order to carry out this project in our own laboratory, we will also need printed wiring PCB layout From lower:
To have a clearer view of this vintage DIY electronic kit, I have attached below a picture with assembled assembly: