Hello,

I am pretty new to trasnsistors and lately I had calculated a ramping circuit using a simple Pn2222 transistor. When the input was 2.0VDC at the base, Vc was 1VoltDC. As the input to the base voltage ramped up towards 3.3 VDC, Vc ramped down to 0.5VDC. Therefore, this circuit had a gain of:

G = Vo/Vin = (1.0 - 0.5) / (3.3-2.0) = 0.3846

However, Now I need a different ratio which is:

G = Vo/Vin = (1.0 - 0.5) / (2.5-2.0) = 1

Therefore in this new circuit I need a ratio 1 : 1. Please view my circuit in attachment. If Vs = 2.0VDC, Vc has to equal to 1VDC. As Vs ramps up to 2.5VDC, VC has to proportionally ramp down to 0.5VDC. Having specified this, I don't understand how to calculate R1 and R2 in order for the transistor to provide this ratio between the input (base) and the output (Vc)?

At these voltages and currents, this transistor's Beta is approx 139 and Vbe is 0.639VDC. Therefore if Vc = 1.0 VDC, we get an Ic of: (3.3-1.0)/2400 = 958.3 ua.

Divided by 139 we get an Ib required of 958.3/139 = 6.894 ua. These currents are pretty exact as I have measured the Vdrop across RB and the ohmic value of RB and calculated the currents.

At 2.0VDC for Vs, Rb should be (2.0 - Vbe) / 6.894ua = 197K

I bleed the 6.894ua through R2 in order to keep my 1VDC at Vc.

Therefore R2 is:

R2 = Vbe/Ib = 0.639/6.894ua = 92K

When I try this circuit with R1 as 200K and R2 as 100K, Vc's ramp is way off...

When Vs = 2.0VDC, Vc = 0.95 VDC. Which is close to 1VDC. But when I ramp Vs up to 2.5VDC, Vc = about 0.84VDC which is not acceptable... as I was expecting 0.5VDC.

Does someone know what I am doing wrong?

Discouraged!

Thanks all.

I am pretty new to trasnsistors and lately I had calculated a ramping circuit using a simple Pn2222 transistor. When the input was 2.0VDC at the base, Vc was 1VoltDC. As the input to the base voltage ramped up towards 3.3 VDC, Vc ramped down to 0.5VDC. Therefore, this circuit had a gain of:

G = Vo/Vin = (1.0 - 0.5) / (3.3-2.0) = 0.3846

However, Now I need a different ratio which is:

G = Vo/Vin = (1.0 - 0.5) / (2.5-2.0) = 1

Therefore in this new circuit I need a ratio 1 : 1. Please view my circuit in attachment. If Vs = 2.0VDC, Vc has to equal to 1VDC. As Vs ramps up to 2.5VDC, VC has to proportionally ramp down to 0.5VDC. Having specified this, I don't understand how to calculate R1 and R2 in order for the transistor to provide this ratio between the input (base) and the output (Vc)?

At these voltages and currents, this transistor's Beta is approx 139 and Vbe is 0.639VDC. Therefore if Vc = 1.0 VDC, we get an Ic of: (3.3-1.0)/2400 = 958.3 ua.

Divided by 139 we get an Ib required of 958.3/139 = 6.894 ua. These currents are pretty exact as I have measured the Vdrop across RB and the ohmic value of RB and calculated the currents.

At 2.0VDC for Vs, Rb should be (2.0 - Vbe) / 6.894ua = 197K

I bleed the 6.894ua through R2 in order to keep my 1VDC at Vc.

Therefore R2 is:

R2 = Vbe/Ib = 0.639/6.894ua = 92K

When I try this circuit with R1 as 200K and R2 as 100K, Vc's ramp is way off...

When Vs = 2.0VDC, Vc = 0.95 VDC. Which is close to 1VDC. But when I ramp Vs up to 2.5VDC, Vc = about 0.84VDC which is not acceptable... as I was expecting 0.5VDC.

Does someone know what I am doing wrong?

Discouraged!

Thanks all.

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