# collector coupled transistor monostable circuit

Discussion in 'The Projects Forum' started by avskalai, Jul 15, 2011.

1. ### avskalai Thread Starter New Member

Jul 15, 2011
2
0
By referring to the values below, I am trying to design a positive edge triggered collector-coupled monostable multi-vibrator using two n-p-n transistors. The circuit is attached below. Please guide me on the necessary calculations needed to make the circuit to work. I need the circuit to produce a pulse of about 1.5ms upon trigger. I did my own calculation. Please correct me if I am wrong. Thanks in advance.

BC237B
Values from datasheet.
VCE (sat) = 0.6V
VBE(sat) = 1.2 V
IC (max) 100 mA
HFE (min) =200

BC327B
Values from datasheet.
VCE (sat) = 0.7V
VBE(sat) = 1.2 V
IC (max) 500 mA
HFE (min) =160

i) Rc = V cc- V ce (sat) / Ic (sat)

= 12V-0.6V/ 50mA
≈ 220Ω

ii) RB2 = V cc- V be(sat) / IB2

iii) IB2(min) = Ic (sat) / h FE(min)

= 50mA/200 = 0.25mA

So in order to saturate Q1, the base current supplied will be 5 x IB2(min)

IB2 = 5 x 0.25mA=1.25mA

So, R1 will be = V cc- V be(sat) / 1.25mA = 12V-1.2V / 1.25mA = 8640Ω

I choosed the nearest value resistor = 10kΩ

T = 0.69RC

So C = 1.5ms / 0.69x10K ≈ 220nF

I need guidance for the next step, that is to calculate the values of R2 and R3. Q1 must be in cut off and only be switched on by a positive edge signal. And also I need an explanation regarding the example circuit below. I need to trigger my circuit in the same way as the example circuit below.

1) What is the purpose of R1, C2, D1,D2 , D3 and D4 ?
: From what I had studied, is that C2 forms a capacitive coupling to prevent the DC voltage from flowing to base of Q1, but allows an AC signal to pass through. By using a simulation software, I observed that there is a voltage spike on the base of Q1 every time the switch is opened.
Diode D2 allows the negative spike to flow through ground preventing the negative spike to flow to the base of Q1.

Am I right? Please correct me if there is a mistake.

Please explain to me how a voltage spike on the base of Q1 turns the transistor on. The spike value on the scope trace is about +2V.

2) Why the voltage value on the base of Q1 is negative in value.
I had set the resistors value based on the example circuit and noticed the voltage on the base of Q1 is negative.

3) Resistor RA and RB forms a voltage divider used to bias Q3.
Am I right?

How to calculate the values for RA and RB based on my first circuit which uses 220Ω resistor as RC?

Expecting some detailed explanations or simply a link for better tutorial or theory/books for better understanding of the transistor circuit theory. ThANKS IN ADVANCE.

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2. ### avskalai Thread Starter New Member

Jul 15, 2011
2
0
Attached is a new schematic diagram. I need guide to calculate the biasing resistor R2 and R3 AND RA, RB.

* The maximum collector current of BC237B based on the datasheet is 100mA. So I will limit the collector current to 50mA.

RC = V cc- V ce (sat) / Ic (sat)
= 12V-0.6V/ 50mA
 220 ohm

*** RC1 = RC2

ii) RB2 = V cc- V be(sat) / IB2; to find IB2 (min)

iii) IB2(min) = Ic (sat) / h FE(min)

= 50mA/200 = 0.25mA

So in order to saturate Q1, the base current supplied will be 5 x IB2(min) ; using a general guideline of supplying 5 times the minimum base current to ensure saturation.

IB2 = 5 x 0.25mA=1.25mA

So, R1 will be = V cc- V be(sat) / 1.25mA = 12V-1.2V / 1.25mA = 8640Ω

I choosed the nearest value resistor = 10kΩ

T = 0.69RC

So C = 1.5ms / 0.69x10K ≈ 220nF

RC2 is of the same value as RC1. But 2 resistors are to used to form a voltage divider to bias Q3. The data values for BC327B is attached above. How the calculate the values of these resistors?