I am using an Arduino Uno board and R-2R DAC to obtain a signal which should vibrate a transducer connected across RL of common collector amplifier using TIP31C. I need around 300mA of current to vibrate the transducer which is much like speaker (resistance is about 10 ohms).
I have calculated the biasing resistor values, by using the link from guitarscience.net,(sorry forum is not allowing to insert the link) as R1=39ohm and R2=82ohms with Vcc=5Volts for all the components. But I dont get the waveform shown in the fig (1) instead output is always HIGH. I have measured the voltages for the above values using multimeter and they are, VBE = 3.7V, VCE = 1.8V, VRL(voltage across the load) = 3V
Then used the link available from hyperphysics (sorry forum is not allowing to insert the link) and used R1=31 kilo ohms and R2=55 kilo ohms upon measuring VBE = 0.5V, VCE = 4.8V, VRL(voltage across the load) = 0.4V and this time output is very much suppresed but has the same shape of the input.
My first constraint is I have to work with 5 Volts for all the components.So arduino, LM358 and TIP31 are all at 5volts supply.
The previous ckt was incomplete and the complete ckt is as below (Fig 3), here the R1 to R11 = 20K ohms and R12 to R18 are 10K ohms and R19 is the load (of about 10 ohms). Upon measuring without connecting the transistor, i.e. with only arduino, R2R and LM358, I am getting 2.5 Volts at the point 'A' as well as 'B'.(seems to be working properly).
But when BJT anlong with the load is connected across TIP 31C (removed biasing resistors after the previous reply), the output at 'A' = 2.5Volts and But at 'B'=0.5 volts, VBE=0.5 Volts, VCE=5Volts, VRL=0 Volts.