Supply & Bias Voltages
Resistors Network
Transistor Specs & Capacitors
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RESULTS
Amplifier Voltage Gain (A)
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Input Resistance (RIN)
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First Pole Frequency (fp1)
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Second Pole Frequency (fp2)
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Base-Emitter Cap (CBE)
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Collector Voltage (VC)
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Collector Current (IC)
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BJT Input Resistance (Rπ)
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Internal Emitter Resistance (re)
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Input Parameters Specification
Supply Rails (VP)The positive voltage rail powering the collector resistors and the base-bias networks.
Collector Resistor (RC)Determines the voltage-drop and sets output-referred gain boundaries.
Feedback LoopsC3 bypasses AC signals to establish single-ended AC gain loops.
Source Resistor (RS)Internal impedance of the active analog signal generator source.
Practical Operational Examples
Small Signal Preamplification
Analyze voltage-follower behaviors when matching extremely small signal inputs with minimal drift.
Wideband Signal Isolation
Plan coupling capacitor configurations (C1/C2) to establish targeted low frequency cutoffs without signal degradation.
Differential Stage Alignment
Verify collector operating currents (IC) to ensure stable symmetrical operation under thermal loads.
Frequency Sweep Planning
Evaluate low-frequency response boundaries to protect analogue mixers from signal loading.
Diagrams & Theory
A wideband differential amplifier utilizes two balanced transistor stages to amplify the difference between inputs. By bypassing the base of Q2 using capacitor C3, the stage functions as a single-ended high-gain active voltage amplifier stage.
Formulas & Mathematical Logic
Quiescent Base Potential: VB = VP * R2 / (R1 + R2)
Emitter potential: VE = VB - VBE
Quiescent Collector Current: IC2 = (VE / (2 * RE)) * (Beta / (Beta + 1))
Active Transconductance: gm = IC2 (mA) / 25 mV
AC Voltage Gain: Av = (gm * (RC || RL)) / 2
How to Use This Calculator
Enter your bias supply voltage (VP) and BJT silicon forward drop (VBE, typically 0.7V).
Enter the resistance values for your bias network (R1, R2, RC, RE) and coupling parameters (RS, RL).
Configure the active current gain bandwidth product (FT) and feedback capacitance (CCB).
Click the orange **CALCULATE** button. Results for quiescent point and high-frequency limits will update below.
About This Calculator
Optimize quiescent point parameters and low-frequency cutoffs for active BJT differential amplifiers.
The CalcBoy Wideband Differential Amplifier Calculator evaluates high frequency poles (fp1, fp2) and computes the optimal 8-bit modulation key (UxMCTL) to keep bit errors to a minimum.
Differential amplifiers are key analog building blocks. They amplify the difference between two input signals while rejecting common-mode noise. In this specific single-ended configuration, bypassing the base of Q2 using C3 creates an asymmetric high-gain AC path, ideal for high-speed signal processing.
This calculator determines the DC operating point using base divider networks and emitter-current splitting. It then applies small-signal BJT models to calculate the active AC voltage gain (Av). It also evaluates the dominant low-frequency cutoff pole (fL) formed by the coupling and bypass capacitors, ensuring flat frequency responses across broad bands.
Gain SymmetrySymmetrical bias current division maintains linear dynamic range margins.
Bypass PerformanceC3 must be sufficiently large to hold the base of Q2 at a stable AC ground potential.
Ideal ApplicationWideband receiver front-ends, preamplifiers, and instrumentation amplifier stages.
Bias Design TipEnsure the collector voltage VC remains above the emitter voltage VE to prevent saturation.
Note: Real-world BJT transconductance (gm) scales with temperature. The equations assume standard room temperature (25°C, where Thermal Voltage Vt ≈ 25mV).
Frequently Asked Questions
1. Why is the AC gain of this single-ended differential amplifier divided by 2?
In a balanced differential pair, the input voltage divides equally between the base-emitter junctions of Q1 and Q2. As a result, only half of the input signal is applied to the active common-base stage (Q2), halving the overall single-ended voltage gain compared to a standard common-emitter amplifier.
2. How does the bypass capacitor C3 affect the frequency response?
C3 acts as an AC short circuit, keeping the base of Q2 at a stable AC ground. At low frequencies where the reactance of C3 increases, Q2 is no longer bypassed to ground, which introduces negative feedback and reduces the low-frequency gain.
3. Why is the input impedance Rin of the differential pair higher than a standard common-emitter stage?
Because the input signal sees the base-emitter junction of Q1 in series with the base-emitter junction of Q2. This doubles the effective input resistance of the active transistors to approximately 2*rπ, boosting the overall input impedance.
4. How is the low-frequency cutoff fL calculated?
The calculator evaluates the three high-pass poles formed by C1 (with Rin + RS), C2 (with RC + RL), and C3 (with the impedance at the base of Q2). It then uses a root-sum-square approximation to find the overall -3dB low-frequency cutoff frequency.
5. What is the typical current gain (Beta) for small-signal RF transistors?
Small-signal RF transistors (such as the 2N2222 or BC547) typically exhibit current gains (Beta) ranging from 100 to 300 under standard operating conditions.
6. How can I increase the high-frequency bandwidth of this amplifier?
To maximize high-frequency bandwidth, use high-speed RF transistors with low parasitic collector-base junction capacitance (Cob), and keep collector resistor RC values relatively small to minimize the output RC time constant.
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