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BERT and Encoder Models — Complete Guide

Deep LearningTransformers🟢 Free Lesson

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Deep Learning

BERT — How Google Changed Search with Bidirectional Understanding

Understand how BERT revolutionized NLP by processing text bidirectionally for better comprehension.

  • Bidirectional context — understand words from both directions
  • Pre-training + fine-tuning — powerful transfer learning paradigm
  • Search and Q and A — transformed Google Search and beyond

Understanding context is the key to understanding language.

BERT and Encoder Models — Complete Guide

BERT (Bidirectional Encoder Representations from Transformers, Devlin et al., 2018) revolutionized NLP by introducing bidirectional pre-training on large unlabeled text, followed by task-specific fine-tuning. It demonstrated that pre-training + fine-tuning outperforms training from scratch on nearly every NLP benchmark.


BERT Architecture

BERT Architecture (Encoder-Only Transformer)Input Tokens[CLS]Thecatsat[MASK]on[SEP]Token + Position + SegmentEmbeddings (d=768)TransformerEncoder × 12Layer 1 → Layer 2 → ... → Layer 12Each: MHA + FFN + LayerNormOutputh₁...hₙ768-dim eachFine-tuning Tasks[CLS] → FCClassificationSentiment, NLIhᵢ → FCToken-levelNER, POShᵢ → start/endSpan ExtractionSQuAD QABERT Model VariantsBERT-base:12 layers, 768 hidden, 12 heads, 110M paramsBERT-large:24 layers, 1024 hidden, 16 heads, 340M paramsImprovements:RoBERTa (better training), ALBERT (parameter sharing), DeBERTa (disentangled attention, SOTA)Efficient:DistilBERT (97% accuracy, 60% faster), TinyBERT (7.5M params, distillation)

Pre-training Objectives

BERT Pre-training ObjectivesTask 1: Masked Language Modeling (MLM)Input: "The [MASK] sat on the [MASK]"Target: "The cat sat on the mat"• Randomly mask 15% of tokens (not all — must prevent shortcut learning)• Of masked: 80% replaced with [MASK], 10% random, 10% unchanged (reduces pre-train/fine-tune mismatch)• Loss: Cross-entropy over vocabulary at masked positions only — forces bidirectional understandingTask 2: Next Sentence Prediction (NSP)Positive: "[CLS] The cat sat [SEP] It was happy [SEP]" → IsNextNegative: "[CLS] The cat sat [SEP] The sky blue [SEP]" → NotNext• 50% positive (actual consecutive sentences), 50% negative (random pair)• Binary classification on [CLS] token output• Note: RoBERTa showed NSP is not helpful — replaced with sentence ordering instead

Fine-tuning BERT

BERT Fine-tuning ProcessPre-trained BERT12-24 layers110-340M paramsLearned: syntax,semantics, worldknowledgeAdd Task HeadClassification: FC层NER: FC per tokenQA: start/end FC~0.1-1M new paramsFine-tuneALL params updatedSmall LR: 2e-5 to 5e-53-5 epochsHours on single GPUFine-tunedModelTask-specificBERT + headReady for inferenceKey insight: Pre-training learns general representations → Fine-tuning adapts to specific task. This transfer learning paradigm achieved SOTA on 11 NLP benchmarks simultaneously.

BERT for Different Tasks


Fine-Tuning BERT


Key Takeaways


What to Learn Next

-> GPT Architecture Compare with autoregressive models.

-> Transformers Master the underlying architecture.

-> NLP Fundamentals Learn natural language processing basics.

-> Transfer Learning Apply pre-trained models to new tasks.

-> Pre-training Language Models Understand how models learn from text.

-> Tokenization for LLMs Learn how text is converted to tokens.

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