LTSpice Beginner Question:BJT+OPAmp

[pharoaz]

Hey, I have the following circuit with the goal of getting a 2.5 volt output and sufficient current to drive a transducer, I have the following in LTspice, however the inverting input to the op amp doesn't equal the non-inverting input of 2.5V, and with V_out = 9.77V, please advise!

The results of the DC operating point analysis are below,

       --- Operating Point ---

V(ref_2v5):    2.5    voltage
V(n004):    2.5    voltage
V(n002):    9.77234    voltage
V(ref5v):    5    voltage
V(neg_ref5v):    -5    voltage
V(n003):    -4.11005    voltage
V(n001):    5    voltage
V(p001):    -4.11005    voltage
V(v_out):    9.77234    voltage
Ic(Pmbt3904):    9.11015e-12    device_current
Ib(Pmbt3904):    -2.29925e-11    device_current
Ie(Pmbt3904):    1.38824e-11    device_current
I(C1):    -1.38824e-21    device_current
I(R1):    9.61514e-10    device_current
I(R2):    9.11015e-12    device_current
I(R3):    2.29925e-11    device_current
I(R4):    -1.38824e-11    device_current
I(V1):    9.61514e-10    device_current
I(V2):    -0.0003    device_current
I(V3):    0.0003    device_current
Ix(u1:1):    -9.61551e-10    subckt_current
Ix(u1:2):    -1.38823e-11    subckt_current
Ix(u1:99):    0.0003    subckt_current
Ix(u1:50):    -0.0003    subckt_current
Ix(u1:34):    2.22045e-11    subckt_current

[mawyatt]

Add a shunt to ground load resistor (100K) as the transistor has no place for its emitter current to go unless a load it attached, then see if the voltage make sense.

Also what model details for the op-amp, if its behavioral then it can produce unrealistic node voltages/currents.

Best,

[magic]

Just came to say the same. Note that Ib is slightly negative, which means current flowing out of the base - this means the base is reverse biased and the opamp tries to pull it down, but this doesn't have much effect on NPN.

The unlabeled node at -4.1V is probably the output of the opamp. Hint: always look at the outputs and inputs of each individual component, try to figure out which components work as expected and which are doing something weird.

[pharoaz]

yep it worked thanks a lot, appreciate it! Btw do you guys recommend any resources for analog circuit designs for beginners?

[mawyatt]

The 1st clue that your circuit is unrealistic is the it can't produce ~9.7V when only a maximum 5V is available

Poor SPICE models, especially Behavioral, are notorious for giving bogus results, sometime blatant as here, but sometimes very subtle!!

Best,

[MathWizard]

So you want 2.5V out, what current will the transducer draw ? In these circuits there will be bias currents and offset voltages. And as someone just learning about this stuff, just those 2 things alone make a big difference to some DC circuits, when compared to ideal op-amps.

There's lots of course pdf's on op-amp circuits, ideal ones and non-ideal. Some good video's on youtube too.

Today I should make a proper test-jig for some op-amp DC-parameters, I just want to fit a few test circuits on a proto-board, some are easier/quicker than others.

[Calvin]

Hi,

the circuit misses on:
- a gnd node as reference for LTspice ... LTSpice doesn´t like floating circuits
- misses on a load (connected to gnd)
- misses on a resistor from the OPAmps In+  to gnd as a dc-input-bias-path

regards
Calvin