# Two-stage transistor amplifier

(sorry for the wall of text, I tried to give as much Info as possible)

I am pretty new to electronics and am working on a project regarding controlling the flow of a pump (BLDC based). End goal: Apply a sinusoidal supply around a variable DC offset with variable amplitude (at least +-4V) and frequency (0-10 Hz, low Freq). One big hurdle is definetly the pump... The manufacturer only provides a very short explaination of the electrical aspects of the pump. Unfortunately, I HAVE to use this model.. Datasheet (I am using M400-S180): https://docs.rs-online.com/9836/0900766b8162656f.pdf. Currently I am using the XR2206 kit for the modulated sine wave. It has a DC Bias of 6V with an Amplitude of +-4V (8Vpp). But it only gives out 20 mA which is not enough to power the pump (I measured the internal Resistance of the pump: 2V - 0,09 A - 22 Ohms, so I guess it needs at least 0,1 A to start). So I wanted to use an bjt common collector amplifier (PNP) to boost the current. I watched some videos and tried to calculate the right resistor values. Basically I assumed Beta=10 and wanted Ie=200mA so Ib=20mA.I am not sure how to calculate Resistances with AC so I assumed to use Vrms? Vmax=10V -> Vrms=7V, R1=7V/20mA = 350 Ohms (used 330). R2=7V/200mA= 35 Ohms. At the time I did not have 35 Ohms at hand so I used the next smallest, 17.5 Ohms so double the current. It worked okayish. The pump started and amp/freq modulation was possible via the XR2206. But the max Vpp changed from 8 Vpp (+-4V) to 3 Vpp (+-1.5V) and also the 17,5 Ohm resistor got pretty hot because of the large current. Also, the Vrms of the Output was only 5V. I am not quite sure why the amp dropped but maybe its because of the difference in output impedance of circuit and input impedance of the pump? (I am honestly a quite a bit confused by impedances so this could be complete rubbish). Also the circuit was not so good designed as I found out through more videos.

I now tried on using a two stage amplifier: Common Emitter (NPN 2N3904) + Common Collector (PNP BC640). I used a better more stable circuit and calculated the Common Emitter circuit values. C1=10uF (AC coupling, I tested with an Osci so no calculation here). According to the tutorials I_R1 = I_R2 >>I_B and I_C = I_E. I wanted V_E = 0.6V. V_B = V_BE+V_E = 1.2V. I_E = 50mA and Beta = 100 according to the Datasheet of the NPN, so I_B = I_E/100. I_R1 should be some 50 times higher than I_B (so we have a stable amplification), I_R1 = 0,012A. Lastly R1 = 1000Ohms, R2 = 100Ohms. V_E=1/2 Supply = 600mV, and with I_E = 50mA; R_E = 10Ohms. Vpp_out=10V from Vpp_in=8V. I could not test it with the pump because the current was not amplified yet. So I now tried combining both amplifier with a new Common Collector circuit. Spoilers: It did not work. So I left the Common Emitter as was and said I wanted I_E2 to be at least 120mA. R7 = 12V/120mA = 100Ohms. I_B2 = I_E2/100 = 1,2mA. I_R3 = I_B2*50 = 60mA. R_3+R_4 = 12v/60mA = 200Ohms. Oh.. pretty low, so I settled for I_R3 = 48mA, R3+R4 = 250Ohms so R3 = 220 Ohms R4 = 47 Ohms. Well as I already teasered it did not work. Amp Modulation was reduced to +-1V (2Vpp). The pump did turn on but the whole circuit drew more than the 120mA calculated, It was around 300mA. Since then I have tried numerous simulations on LTSpice so I dont have to build up/down circuits every 10 mins but until now no success. Can anyone suggest me a direction where I can head to or point out the flaws in my circuit/calc? Does anyone have a clue why the amplitude of the sine wave dropped? Also: Sorry for the Variable-formatting, pretty messy!

• Why the discrete transistor amplifiers? In my view using an opamp based amplifier is much more easy to design and will give a more predictable result. – Bimpelrekkie Oct 26 '20 at 10:29
• No particular reason. I just searched for an amplifier circuit and invested myself for transistors. In your opinion, would an op-amp based amplifier be better suited in order to get the characteristics for the pump/bldc motor? – MagoAbd Oct 26 '20 at 12:40
• @Bimpelrekkie I researched opamp based current amplifier and tbh my search was not very fruitful. Could you recommend some links where a beginner could start? – MagoAbd Oct 26 '20 at 14:18
• This is a good read when starting with opamps: web.mit.edu/6.101/www/reference/op_amps_everyone.pdf – Bimpelrekkie Oct 26 '20 at 14:19
• Thanks a lot @Bimpelrekkie! – MagoAbd Oct 26 '20 at 14:28

## 1 Answer

The load on the amplifier (first transistor) collector cannot be larger than 47 ohms.

Thus the maximum gain cannot be larger than 47/10, at the largest.

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View that first transistor as being a voltage_controlled current generator.

The load on that first transistor collector is what converts the collector current back into a voltage.

The various paralleled resistances on that collector are

• 47 ohms to ground

• 220 to rail on the collector of first transistor

• 220 to rail on base of 2nd transistor

• R_early of the first transistor

• R_in (approximatly beta * Rload) of 2nd transistor

And all these in parallel can only be LESS than the smallest, which is 47 ohms

• Could you explain how you came to this? Do you mean that my "final load", i.e. the pump, cannot be higher than 47 ohms or the that 2nd amplifier circuit cannot have a total resistance larger than 47 ohms? And how do I use this constraint in my circuit design? – MagoAbd Oct 26 '20 at 12:42